Catalytic materials, photoanodes, and photoelectrochemical cells for water electrolysis and other, electrochemical techniques

ABSTRACT

Catalytic materials, photoanodes, and systems for electrolysis and/or formation of water are provided which can be used for energy storage, particularly in the area of solar energy conversion, and/or production of oxygen and/or hydrogen. Compositions and methods for forming photoanodes and other devices are also provided.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/103,905, filed Oct. 8, 2008, entitled “CatalystCompositions and Photoanodes for Photosynthesis Replication and OtherPhotoelectrochemical Techniques,” by Nocera, et al., and U.S.Provisional Patent Application Ser. No. 61/187,995, filed Jun. 17, 2009,entitled “Catalytic Materials, Photoanodes, and Systems for WaterElectrolysis and Other Electrochemical Techniques,” by Nocera, et al.,each herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with the support under the following governmentcontract F32GM07782903 awarded by the National Institutes of Health andCHE-0533150 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to photoanodes for electrolysis of waterwhich can be used for energy storage. The invention also relates tocompositions and methods for forming a photoanode. In some embodiments,electrochemical devices such as photoelectrochemical devices areprovided for the catalytic formation of oxygen gas and/or hydrogen gasfrom water.

BACKGROUND OF THE INVENTION

Solar energy can be considered to be a carbon-neutral energy source ofsufficient scale to meet future global energy demand. The diurnalvariation in local insolation, however, requires a cost-effectivestorage of solar energy for its large scale deployment as a primaryenergy source. In nature, photosynthesis captures sunlight and convertsit into a wireless current which is stored. An approach to duplicatingnatural photosynthesis outside of a leaf is to capture and convert solarlight into spatially separated electron/hole pairs within aphotoelectrochemical cell. Photoelectrochemical devices may be used toproduce hydrogen and oxygen gases from water. Photoelectrochemicaldevices utilize solar energy for the electrolysis of water, andgenerally employ a photoactive electrode, which, upon exposure tosunlight, produces electron/hole pairs that may be used for theelectrolysis of water to produce hydrogen and/or oxygen gases. The netresult is the storage of solar energy in the chemical bonds of H₂ andO₂.

In order to store energy via electrolysis, catalysts are required whichefficiently mediate the bond rearranging “water splitting” reaction. Thestandard reduction potentials for the O₂/H₂O and H⁺/H₂ half-cells aregiven by Equation 1 and Equation 2.

$\frac{\begin{matrix}{ {O_{2} + {4H^{+}4e^{-}}}rightarrow{H_{2}O} \mspace{34mu}} & {\mspace{11mu} {E^{0} = {{+ 1.23} - {0.059({pH})V\mspace{76mu} (1)}}}\mspace{14mu}} \\{ {2H_{2}}rightarrow{{4H^{+}} + {4e^{-}}} \;} & {E^{0} = {0.00 - {0.059({pH})V\mspace{95mu} (2)}}}\end{matrix}}{ {{2H_{2}} + O_{2}}rightarrow{2H_{2}O} \mspace{405mu}}$

For a catalyst to be efficient for this conversion, the catalyst shouldoperate at voltages close to the thermodynamic value of each halfreaction, which are defined by half-cell potentials, E^(o). Voltage inaddition to E^(o) that is required to attain a given catalytic activity,referred to as overpotential, limits the energy conversion efficiency.Considerable effort has been expended by many researchers in efforts toreduce overpotential in this reaction. It may be considered that oxygengas production from water at low overpotential and under benignconditions using catalytic materials composed of earth-abundantmaterials presents the greatest challenge to water electrolysis. Theoxidation of water to form oxygen gas requires the coupled transfer offour electrons and four protons to avoid high-energy intermediates. Inaddition to controlling multi-proton-coupled electron transferreactions, a catalyst, in some cases, should also be able to tolerateprolonged exposure to oxidizing conditions.

While photoelectrochemical devices and photoanodes exist for theelectrolysis of water, these devices are generally composed of expensivematerials and/or operate with low energy conversion efficiencies.Therefore, a need remains for the development of improved materials anddevices that operate with increased energy conversion efficiency.

SUMMARY OF THE INVENTION

The present invention relates to catalytic materials for electrolysis ofwater that can be used for energy storage, particularly in the area ofsolar energy conversion. The invention also relates to compositions andmethods for forming a photoanode. In some embodiments,photoelectrochemical devices are provided for the catalytic formation ofoxygen gas from water. The subject matter of the present inventioninvolves, in some cases, interrelated products, alternative solutions toa particular problem, and/or a plurality of different uses of one ormore systems and/or articles.

In one aspect, the invention is directed to a method. According to afirst embodiment, a method for forming a photoanode for the catalyticproduction of oxygen from water comprises providing a solutioncomprising metal ionic species and anionic species, providing aphotoactive electrode comprising a photoactive composition and aphotosensitizing agent, and causing the metal ionic species and theanionic species to form a catalytic material associated with thephotoactive electrode by application of a voltage to the photoactiveelectrode.

According to another embodiment, a method for producing oxygen fromwater, comprises the steps of providing a photoelectrochemical cellcomprising a photoactive electrode comprising a photoactive compositionand a photosensitizing agent, an electrolyte, and a catalytic materialintegrally connected to the photosensitizing agent, the catalyticmaterial comprising metal ionic species and anionic species, and whereinthe catalytic material does not consist essentially of a metal oxide ormetal hydroxide, and illuminating the photoelectrochemical cell withlight to thereby produce oxygen gas from water.

In another aspect, the invention is directed towards photoanodes.According to a first embodiment, a photoanode for the catalyticproduction of oxygen from water, comprises a photoactive electrodecomprising a photoactive composition and a photosensitizing agent, and acatalytic material associated with the photoactive electrode, comprisingcobalt ions and anionic species comprising phosphorus.

According to another embodiment, a photoanode for the catalyticproduction of oxygen from water comprises a photoactive electrodecomprising a photoactive composition and a photosensitizing agent, and acatalytic material comprising metal ionic species and anionic species,wherein the metal ionic species with an oxidation state of (n+x) and theanionic species define a substantially non-crystalline composition andhave a K_(sp) value which is less, by a factor of at least 10³, than theK_(sp) value of a composition comprising the metal ionic species with anoxidation state of (n) and the anionic species.

According to yet another embodiment, a photoanode for the catalyticproduction of oxygen from water comprises a photoactive electrodecomprising a photosensitizing agent and a photoactive composition, and acatalytic material comprising metal ionic species and anionic species,wherein the catalytic material is formed by application of a voltage toa photoactive electrode.

In yet another aspect, the invention relates to photoelectrochemicalcells. According to a first embodiment, a photoelectrochemical cellcomprises a photoanode comprising a photoactive electrode comprising andphotoactive composition and a photosensitizing agent, and a catalyticmaterial comprising metal ionic species and anionic species, wherein thecatalytic material does not consist essentially of a metal oxide ormetal hydroxide, at least one second electrode, and an electrolyte,wherein the photoelectrochemical cell is capable of producing oxygen gasfrom water.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. Unless indicated asrepresenting the prior art, the figures represent aspects of theinvention. In the figures, each identical or nearly identical componentillustrated is typically represented by a single numeral. For purposesof clarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. In the figures:

FIG. 1 illustrates a non-limiting example of a photoelectrochemicalcell, according to one embodiment.

FIG. 2 illustrates an energy diagram of a photoelectrochemical devicecomprising a photoactive electrode and an electrode, wherein thephotoactive electrode is biased positively with respect to theelectrode, according to one embodiment.

FIG. 3 illustrates an energy diagram of a photoelectrochemical cellcomprising a first photoactive electrode and a second photoactiveelectrode, wherein the first photoactive electrode is biased positivelywith respect to the second photoactive electrode, according to oneembodiment.

FIGS. 4A-4B illustrate the formation of a photoanode, according to oneembodiment.

FIGS. 5A-5D illustrate examples of how a composition may associate witha photoactive electrode upon application of a voltage (e.g., aphotovoltage) to the photoactive electrode, according to someembodiments.

FIG. 6 illustrates an energy diagram for a photoelectrochemical devicecomprising a photoactive electrode and an electrode, wherein thephotoactive electrode is associated with a dye, according to oneembodiment.

FIGS. 7A-7E illustrate the formation of a catalytic material on aphotoactive electrode, according to one embodiment.

FIGS. 8A-8C illustrate a non-limiting example of a dynamic equilibriumof a catalytic material, according to one embodiment.

FIGS. 9A-9C represent an illustrative example of changes in oxidationstate that may occur for a single metal ionic species during a dynamicequilibrium of an electrode, according to one embodiment, during use.

FIG. 10 shows a non-limiting embodiment of a hybrid photoelectrochemicalcell.

FIG. 11 shows a non-limiting example of an electrochemical device.

FIG. 12 shows a non-limiting embodiment of a bi-photoelectrochemicalcell.

FIG. 13 shows another non-limiting embodiment of abi-photoelectrochemical cell.

FIG. 14 shows a SEM image of a catalytic material comprising cobaltelectrodeposited onto a thin film of CdS, according to a non-limitingembodiment.

FIG. 15 shows SEM images of a thin film CdS electrode treated with asolution of 0.1M MePi (pH 8.5) and 2 mM Co²⁺ in the (A) presence and (B)absence of irradiation with visible light, according to a non-limitingembodiment.

FIG. 16 shows the band edge positions of various forms of TiO₂ alongwith the standard reduction potential of the hydroxyl radical and thepotential for operation of the CoPi catalyst, according to someembodiments.

FIGS. 17A-17F illustrate non-limiting examples of photoelectrochemicalcells.

Other aspects, embodiments, and features of the invention will becomeapparent from the following detailed description when considered inconjunction with the accompanying drawings. The accompanying figures areschematic and are not intended to be drawn to scale. For purposes ofclarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. All patent applications and patentsincorporated herein by reference are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control.

DETAILED DESCRIPTION

The present invention relates to photoanodes for electrolysis of waterwhich can be used for energy storage. The invention also relates tocompositions and methods for forming a photoanode. In some embodiments,photoelectrochemical devices are provided for the catalytic formation ofoxygen gas and/or hydrogen gas from water. The invention allows for thefacile, low-energy conversion of water to hydrogen gas and/or oxygen gasusing a photoactive electrode. In some cases, the conversion may bedriven by exposing the photoactive electrode to electromagneticradiation (e.g., sunlight). Energy can be stored, via the catalyticmaterial of the invention, in the form of oxygen gas and hydrogen gas.The hydrogen and oxygen gases can be recombined at any time, forexample, later as a stored source of energy, whereby they form water andrelease significant energy that can be captured in the form ofmechanical energy, electricity, or the like. In other cases, thehydrogen and/or oxygen gases may be used in another process.

According to some embodiments, compositions and methods for forming aphotoanode are provided. In some cases, a photoanode may catalyticallyproduce oxygen gas from water. As shown in Equation 1, water may besplit to form oxygen gas, electrons, and hydrogen ions. Although it neednot be, a photoanode of the invention can be operated in benignconditions (e.g., neutral or near-neutral pH, ambient temperature,ambient pressure, etc.). A photoanode may comprise a photoactiveelectrode, metal ionic species and anionic species, wherein the metalionic species and anionic species are associated with the photoactiveelectrode. The metal ionic species and anionic species may be selectedsuch that, when exposed to an aqueous solution (e.g., an electrolyte),the metal ionic species and anionic species are in dynamic equilibriumwith the aqueous solution, as described herein.

Photoanodes of the present invention, in some cases, comprise acatalytic material. Many species of the class of catalytic materialprovided by the invention are made of readily-available, low-costmaterial, and are easy to make. Accordingly, the invention has thepotential to dramatically change the field of solar energy capture,storage, and use. The subject matter of the present invention involves,in some cases, interrelated products, alternative solutions to aparticular problem, and/or a plurality of different uses of one or moresystems and/or articles.

In all descriptions of the use of water (e.g., for the production ofoxygen gas) for catalysis herein, it is to be understood that the watermay be provided in a liquid and/or gaseous state. The water used may berelatively pure, but need not be, and it is one advantage of theinvention that relatively impure water can be used. The water providedcan contain, for example, at least one impurity (e.g., halide ions suchas chloride ions). In some cases, the device may be used fordesalination of water. It should be understood that while much of theapplication herein focuses on the catalytic formation of oxygen gas fromwater, this is by no means limiting, and the compositions, photoanode,methods, and/or systems described herein may be used for other catalyticpurposes, as described herein.

In some embodiments, photoanodes are provided which may produce oxygengas from water. As shown in Equation 1, water may be split to formoxygen gas, electrons, and hydrogen ions. Although it need not be, anelectrode may be operated in benign conditions (e.g., neutral ornear-neutral pH, ambient temperature, ambient pressure, etc.). In somecases, the electrodes described herein operate catalytically. That is,an electrode may be able to catalytically produce oxygen gas from water,but the electrode may not necessarily participate in the relatedchemical reactions such that it is consumed to any appreciable degree.Those of ordinary skill in the art will understand the meaning of“catalytically” in this context. An electrode may also be used for thecatalytic production of other gases and/or materials.

As used herein, a photoanode is a photoactive electrode, in addition toany catalytic material adsorbed thereto. In some embodiments, thephotoactive electrode may comprise a photoactive composition and aphotosensitizing agent. The catalytic material may comprise metal ionicspecies and anionic species, wherein the metal ionic species and anionicspecies are associated with the photoactive electrode. The metal ionicspecies and anionic species may be selected such that, when exposed toan aqueous solution (e.g., an electrolyte or water source), the metalionic species and anionic species are in dynamic equilibrium with theaqueous solution, as described herein.

In some embodiments, a photoanode of the present invention comprises aphotoactive electrode and a catalytic material associated with thephotoactive electrode. A “catalytic material” as used herein, means amaterial that is involved in and increases the rate of a chemicalelectrolysis reaction (or other electrochemical reaction) and which,itself, undergoes reaction as part of the electrolysis, but is largelyunconsumed by the reaction itself, and may participate in multiplechemical transformations. A catalytic material may also be referred toas a catalyst and/or a catalyst composition. A catalytic material is notsimply a bulk photoactive electrode material which provides and/orreceives electrons from an electrolysis reaction, but a material whichundergoes a change in chemical state of at least one ion during thecatalytic process. For example, a catalytic material might involve ametal center which undergoes a change from one oxidation state toanother during the catalytic process. In another example, the catalyticmaterial might involve metal ionic species which bind to one or moreoxygen atoms from water and release the oxygen atoms as dioxygen (i.e.,O₂). Thus, catalytic material is given its ordinary meaning in the fieldin connection with this invention. As will be understood from otherdescriptions herein, a catalytic material of the invention that may beconsumed in slight quantities during some uses and may be, in manyembodiments, regenerated to its original chemical state.

“Electrolysis,” as used herein, refers to the use of an electric currentto drive an otherwise non-spontaneous chemical reaction. For example, insome cases, electrolysis may involve a change in redox state of at leastone species and/or formation and/or breaking of at least one chemicalbond, by the application of an electric current. Electrolysis of water,as provided by the invention, can involve splitting water into oxygengas and hydrogen gas, or oxygen gas and another hydrogen-containingspecies, or hydrogen gas and another oxygen-containing species, or acombination.

In some embodiments, methods are provided for forming a photoanodecomprising a photoactive electrode (e.g., an n-type semiconductorphotoactive material), metal ionic species, and anionic species. Thephotoanode may be formed by exposing a photoactive electrode to asolution comprising metal ionic species and anionic species, followed byapplication of a voltage to the photoactive electrode. The term“application of a voltage,” as used herein, in some embodiments, issynonymous with the term formation of a photovoltage (e.g., formation ofelectron/hole pairs in a material by exposing the material toelectromagnetic radiation). For example, the voltage may be applied to aphotoactive electrode by an external power source (e.g., a battery) orby exposing a photoactive electrode to electromagnetic radiation (e.g.,sunlight, to produce a photovoltage), as described herein. The metalionic species and anionic species may associate with the photoactiveelectrode and form a composition (e.g., a catalytic material) associatedwith the photoactive electrode. In some cases, when associating with thephotoactive electrode, the metal ionic species may be oxidized orreduced as compared to the metal ionic species in solution, as describedherein.

In some embodiments, photoelectrochemical devices are provided whichcomprise a photoanode as described herein. In some embodiments,photoelectrochemical devices comprising a photoanode as described hereinare capable of catalytically producing oxygen gas from water. In somecases, the device may additionally produce hydrogen gas. Devices aredescribed herein.

Without wishing to be bound by theory, the devices and methods asdescribed herein may be used for the photoelectrolysis of water andconversion of light to electrical energy, and in some cases, solely usesolar energy (e.g., sunlight) as the power source. For example, aphotoanode as described herein may comprise a catalytic materialassociated with an n-type semiconductor photoactive electrode. When thephotoanode is exposed to light, electrons are excited from the valenceband to the conduction band of the n-type semiconductor, therebycreating holes in the valence band and free electrons in the conductionband. In some embodiments, the excited electron and correspondingelectron-hole may separate spatially within the semiconductor materialfrom the point of generation. Such charge separation may give rise to aphotovoltage within the semiconductor. The electron-holes may betransported to the semiconductor-electrolyte interface, where they mayreact with a water molecule (e.g., via a catalytic material), resultingin the formation of oxygen gas and/or hydrogen ions. The electronsproduced at the photoanode may be conducted by means of an externalelectrical connection to the counter electrode where they may combinewith hydrogen ions of water molecules (or another source such as anacid) in the electrolytic solution to produce hydrogen gas. In instanceswhere the conduction band level of the semiconductor is more negativethan the H₂O/H₂ energy level and the valence band level of thesemiconductor is more positive than the O₂/H₂O energy level, theelectrolysis of water may be accomplished solely through the use ofsolar energy (e.g., without the use of an external power source). Insome cases, association of a catalytic material with a photoactiveelectrode (e.g., a photoanode as described herein) may cause theefficiency and/or yield of the formation of oxygen to increase ascompared to the photoactive electrode itself, when operated underessentially identical conditions, as described herein.

There are many benefits to photoanodes and to the methods for producingphotoanodes as described herein. For example, the method for forming aphotoanode is easily adaptable and may be used to produce photoanodes ofvarying sizes and shapes, as described herein. In addition, thephotoanodes produced by the provided methods can be robust andlong-lived, and can be resistant to poisoning by acidic and/or basicconditions and/or environmental conditions (e.g., the presence of carbonmonoxide). Photoanode poisoning may be thought of as any chemical orphysical change in the status of the photoanode that may diminish orlimit the use of a photoanode in a photoelectrochemical device and/orlead to erroneous measurements. Photoanode poisoning may manifest itselfas the development of unwanted coatings, and/or precipitates, on thephotoanode, or dissolution and/or erosion of the photoanode. Forexample, some photoactive electrodes (e.g., CdS, CdSe, GaAs, GaP) whichmay be employed in a photoanode as described herein, may be subject tosurface reactions in aqueous, acidic, and/or basic conditions. In someembodiments, the presence of the catalytic material associated with thephotoactive electrode may help prevent and/or reduce unwanted surfacereactions as opposed to a photoactive electrode which does not comprisethe catalytic material.

Methods of making a photoanode as described herein also represent asignificant development. In some embodiments, photoanodes are providedwhich are made from materials which are easily, reproductively, andinexpensively made. In some embodiments, a photoanode may be formed byimmersing a photoactive electrode (e.g., hematite, TiO₂, etc.) in asolution comprising metal ionic species and anionic species. Applicationof a voltage to the photoactive electrode may cause the metal ionicspecies and the anionic species to associate with the photoactiveelectrode to form a catalytic material associated with the photoactiveelectrode, thereby forming the photoanode. In some cases, association ofthe metal ionic species with the photoactive electrode may comprise achange in oxidation state of the metal ionic species from (n) to (n+x),wherein x may be 1, 2, 3, and the like.

The invention can also be characterized in terms of performance of thecatalytic material of the invention (and/or photoanode comprising thecatalytic material). One way of doing this, among many, is to comparethe energy conversion efficiency and/or the current density of thephotoanode versus the photoactive electrode alone. Typical photoactiveelectrodes are described more fully herein and may comprise Fe₂O₃, TiO₂,and the like. The photoactive electrode may be able to function, itself,as a catalytic photoanode for water electrolysis, and may have been usedin the past to do so. So, comparison of the energy conversion efficiencyand/or the current density during catalytic water electrolysis (wherethe photoanode catalytically produces oxygen gas from water), using thephotoactive electrode, as compared to essentially identical conditions(with the same counter electrode, same electrolyte, same circuitry, samewater source, etc.), using the photoanode of the invention includingboth photoactive electrode and catalytic material, can be compared. Inmost cases, the energy conversion efficiency and/or the current densityof the photoanode is greater than the energy conversion efficiencyand/or the current density of the photoactive electrode alone, whereeach is tested independently under essentially identical conditions.

In some cases, the energy conversion efficiency of the photoanodecomprising the composition may be at least about 5%, at least about 10%,at least about 15%, at least about 20%, at least about 30%, at leastabout 40%, at least about 50%, at least about 60% at least about 70%, atleast about 80%, at least about 90%, at least about 100%, at least about150%, at least about 200%, or greater, than the efficiency of thephotoactive electrode alone, operated under essentially identicalconditions. The increase in energy conversion efficiency may bedetermined operating a photoanode as described herein (e.g., comprisinga photoactive electrode and catalytic material) and the photoactiveelectrode under essentially identical conditions and comparing theresults. Energy conversion efficiency is the ratio between the usefuloutput of an energy conversion device and the input, in energy terms andtechniques for measuring the efficiency, as will be known to those ofordinary skill in the art. In some cases, the current density of thephotoanode may be greater than the current density of the photoactiveelectrode by a factor of at least about 10, about 100, about 1000, about10⁴, about 10⁵, about 10⁶, about 10⁸, about 10¹⁰, and the like. In someembodiments, the current density of the photoanode may exceed thecurrent density of the photoactive electrode by a factor between about10⁴ and about 10¹⁰, between about 10⁵ and about 10⁹, or between about10⁴ and about 10⁸. The current density may either be the geometriccurrent density or the total current density, as described herein.

In some embodiments, an electrochemical cell comprising at least a firstelectrode, a second electrode, and an electrolyte, wherein at least oneelectrode is capable of converting solar energy into an electrochemicalpotential and used for water electrolysis is provided. In a firstnon-limiting embodiment, the first electrode may comprise an n-typesemiconductor electrode (e.g., comprising a catalytic material) and asecond electrode comprises a conductor (e.g., a metal), wherein thefirst electrode may be biased positively with respect to the secondelectrode. In some cases, the bias may be supplied by a photovoltaiccell. In a second non-limiting embodiment, the first electrode comprisesan n-type semiconductor and the second electrode comprises a p-typesemiconductor (e.g., a tandem configuration), wherein the firstelectrode is biased positively with respect to the second electrode. Ina third non-limiting embodiment, the first electrode comprises aconductor (e.g., a metal) and the second electrode comprises a p-typesemiconductor, wherein the first electrode may be biased positively withrespect to the second electrode.

In one embodiment, the processes that may occur in aphotoelectrochemical cell are as follows. The first electrode may beexposed to electromagnetic radiation, wherein the first electrodecomprises an n-type semiconductor and may be biased positively withrespect to a second electrode. The light may excite the semiconductingmaterial of the first electrode, and result in the formation ofelectronic charged carriers (e.g., electron/hole pairs). Water may beoxidized by the electron holes produced at the first electrode. Thehydrogen ions produced at the first electrode may be transported (e.g.,through the electrolyte) to the second electrode, and the electronsproduced at the first electrode may be transferred to the secondelectrode through an external circuit. The transported hydrogen ions(e.g., H⁺ or another form such as H₂PO₄ ⁻) may be reduced withtransported electrons at the second electrode, thereby forming hydrogengas. FIG. 1 illustrates one possible arrangement of aphotoelectrochemical cell and is described herein.

The photovoltage of the n-type semiconducting material may be related tothe energy of the electromagnetic radiation as well as to the band gapof the material. The band gap of a material is the energy differencebetween the top of the valence band and the bottom of the conductionband, as will be known to those of ordinary skill in the art. If aphoton has energy greater than or equal to the band gap of the material,then electrons can form in the conduction band and holes can form in thevalence band, related by the following Equation 3:

hv→e′+h.  (3)

where h is Planck's constant, v is the frequency of the photon, e′ is anelectron, and h. is an electron hole. Generally, an electric field orbias (e.g., provided through doping of the semiconductor material and/orthrough the application of an external voltage) may be required at thephotoactive electrode/electrolyte interference to avoid recombination ofthe electron and the hole.

In some embodiments, the process that takes place at the first electrodewhich is biased positively with respect to the second electrode is shownin Equation 4.

4h.+H₂O (liquid or gas)→O₂ (gas)+4H⁺  (4)

The process shown in Equation 4, in some cases, may take place at thefirst electrode/electrolyte interface. This process produces oxygen gaswhich may be released, stored, and/or used in various devices/methods.The electrons and the hydrogen ions may combine at the second electrodeto form hydrogen gas, as shown in Equation 5.

2H⁺+2e′→H₂ (gas)  (5)

The overall reaction that takes place is shown in Equation 6.

4hv+H₂O (liquid or gas) O₂→(gas)+2H₂ (gas)  (6)

The overall reaction can occur if the energy of the photons absorbed bythe first electrode is equal to or greater than the threshold energy,E_(t), which are related by Equation 7:

$\begin{matrix}{E_{t} = \frac{\Delta \; G_{H_{2}O}^{o}}{4}} & (7)\end{matrix}$

where ΔG_(H) ₂ _(O) ^(o) is the standard free energy of the reactionshown in Equation 6 (4.92 eV). E_(t) is equal to 1.23 eV and theelectrolysis of water is possible when the electromotive force of thephotoelectrochemical device is equal to or greater than 1.23 eV. Itshould be understood, however, that in some embodiments, the proton maybe associated with a species and may be transported via association witha species in solution (e.g., a buffer species). The thermodynamicsdiscussed above, in most cases, would be applicable in such embodiments.

It should be understood, that photoanodes as described herein may beformed prior to incorporation into a device or may be formed duringoperation of a device. For example, in some cases, a photoanode may beformed using methods described herein (e.g., exposing a photoactiveelectrode to a solution comprising metal ionic species and anionicspecies, followed by application of a voltage to the photoactiveelectrode and association of catalytic material comprising the metalionic species and anionic species with the photoactive electrode). Thephotoanode may then be incorporated into a device. As another example,in some cases, a device may comprise a photoactive electrode, and asolution (e.g., electrolyte) comprising metal ionic species and anionicspecies. Upon operation of the device (e.g., application of a voltage tothe photoactive electrode), a catalytic material (e.g., comprising themetal ionic species and anionic species from the solution) may associatewith the photoactive electrode, thereby forming the photoanode in thedevice. After formation of the photoanode, the photoanode can be usedfor purposes described herein with or without change in environment(e.g., change in solution or other medium to which the electrode isexposed), depending upon the desired formation and/or use medium, whichwould be apparent to those of ordinary skill in the art.

FIG. 2 shows an energy diagram of a photoelectrochemical cell comprisinga photoactive electrode and an electrode, wherein the photoactiveelectrode is biased positively with respect to the electrode andcomprises an n-type semiconductor (e.g., a photoanode). In this figure,E_(F) is the Fermi energy, E_(C) and E_(V) are the energies of thebottom of the conduction band and the top of the valence band,respectively, of the photoanode, and E_(g) is the band gap. For the cellto operate, the oxygen energy levels (O₂/H₂O) should be above thevalence band of the photoanode for electron-hole transfer to occur, andfor the same reason, the hydrogen energy level (H⁺/H₂) should be belowthe Fermi level of the electrode (e.g., when the electrode is aconductor). In some cases, a photoelectrochemical device may require anexternal bias (e.g., voltage) in order for the photoelectrochemicaldevice to operate. Application of an external bias may aid in creatingincreased charge separation between the electron/hole pairs at thephotoanode as compared to a photoanode without a charge bias. In someembodiments, a charge bias of at least about 0.1 V, at least about 0.3V, at least about 0.5 V, at least about 1.0 V, at least about 2.0 V, orgreater, may be provided to the photoelectrochemical device. The chargebias may aid in reducing the probability of recombination of an electronin the conduction band and a hole created in the valence band upon theabsorption of light energy. Some possible arrangements ofphotoelectrochemical devices are described herein.

FIG. 3 shows the energy diagram of a photoelectrochemical devicecomprising a first photoactive electrode (e.g., an n-type semiconductor)and a second photoactive electrode (e.g., a p-type semiconductor). Theintrinsic nature of the band position leads to the first photoactiveelectrode being “biased” positively with respect to the secondphotoactive electrode (although no external bias is provided, e.g., viaa power source). This type of photoelectrochemical device may bereferred to as a bi-photoelectrochemical cell or a tandemphotoelectrochemical cell. In the figure, E_(F) is the Fermi energy,E_(C) and E_(V) are the energies of the bottom of the conduction bandand the top of the valence band, respectively, of the photoactiveelectrodes, and E_(g) is the band gap for each photoactive electrode. Abi-photoelectrochemical cell may be able to operate using only solarenergy without the need for an external bias, e.g., as may be generallyrequired for a photoelectrochemical cell comprising a single photoactiveelectrode. Various possible arrangements for a bi-photoelectrochemicalcell are described herein. Additional devices that may be used incombination with a photoanode are also discussed in more detail below.

In some embodiments, a method of forming a photoanode comprises causingmetal ionic species and anionic species to associate with a photoactiveelectrode by application of a voltage to the photoactive electrode. Insome embodiments, the method may comprise providing a solutioncontaining metal ionic species and anionic species and immersing aphotoactive electrode in the solution, followed by application ofvoltage to the photoactive electrode. A non-limiting example offormation of a photoanode is shown in FIG. 4. FIG. 4A shows a container110 comprising a photoactive electrode 112 and a solution 114 in whichare suspended, but more typically dissolved, metal ionic species 116 andanionic species 118. In some cases, the photoactive electrode is inelectrical communication 120 with a power source (not shown). FIG. 4Bshows the same experimental set-up upon application of voltage to thephotoactive electrode by the power source. In some cases, however,voltage may be applied to the photoactive electrode by exposing thephotoactive electrode to electromagnetic radiation or by an externalpower source (e.g., a battery). Metal ionic species 122 and anionicspecies 124 associate with the photoactive electrode 126 to form acomposition (e.g., a catalytic material) 128 associated with thephotoactive electrode. The catalytic material may be transparent,substantially transparent, substantially opaque, and/or opaque. In aparticular embodiment, the catalytic material is transparent and/orsubstantially transparent.

In some cases, voltage may be applied to a photoactive electrode by apower source. For example, voltage may be applied to the photoactiveelectrode by batteries, power grids, regenerative power supplies (e.g.,wind power generators, photovoltaic cells, tidal energy generators,etc.), generators, and the like. The power source may comprise one ormore of such power supplies (e.g., batteries and a photovoltaic cell).The voltage applied may be AC or DC. In such embodiments, the voltageapplied to the photoactive electrode may be substantially similar to allsurfaces of the area. In some cases, the thickness of the compositionformed on the photoactive electrode is substantially uniform across theareas where the composition is present. For example, as shown in FIG.5A, application of voltage to photoactive electrode 2 through wire 4connected to an outside power source which is immersed in solution 6comprising metal ionic species and anionic species causes composition 8to associate with photoactive electrode 2.

In other cases, voltage (e.g., photovoltage) may be applied to thephotoactive electrode by exposing the photoactive electrode toelectromagnetic radiation (e.g., sunlight). As described herein,application of electromagnetic radiation to a photoactive electrode maycause the production of electron/hole pairs to form (e.g., formation ofa photovoltage). In some instances, the photoactive electrode may beexposed to varying levels of electromagnetic radiation. For example,some surfaces of the photoactive electrode may be exposed to a differingelectromagnetic radiation (e.g., wavelength or range of wavelengths,time of exposure, power (e.g., wattage, etc.)), than other surfaces ofthe photoactive electrode. In some cases, at least a portion of all thesurfaces of the photoactive electrode are exposed to substantiallysimilar electromagnetic radiation. In some cases, surfaces which areexposed to a solution comprising metal ionic species and anionic speciesare exposed to electromagnetic radiation, while in other cases, surfaceswhich are not exposed to a solution comprising metal ionic species andanionic species are exposed to electromagnetic radiation. In someinstances, the thickness of the composition associated with thephotoanode may or may not be substantially similar in the areas of thephotoactive electrode which are exposed to electromagnetic radiation. Insome cases, the areas of the photoactive electrode which are more active(e.g., produce more electron/hole pairs in that area), may comprise acatalytic material which is thicker than the composition in areas whichare less active (e.g., produce less electron/hole pairs in that area).For example, as shown in FIGS. 5B and 5C, exposing photoactive electrode2 to electromagnetic radiation 10 causes composition 8 to associate withphotoanode. In some cases, the composition may associate with only theareas which were directly exposed to light as shown in FIG. 5B (e.g.,surface 12 was exposed to light and is associated with composition 8).In other cases, substantially all surfaces may be associated with thecomposition upon exposure to electromagnetic radiation, as shown in FIG.5C (e.g., surface 12 was exposed to light and both surface 12 and 14 areassociated with composition 8), for example, in instances where thephotoactive electrode is substantially transparent.

In some cases, some areas of the photoactive electrode may bedisproportionately exposed to electromagnetic radiation at a levelgreater than exposure at other areas of the photoactive electrode, suchthat the formation of a composition is greater in areas that wereexposed to greater levels of electromagnetic radiation than areas thatreceived less electromagnetic radiation. For example, in someembodiments, the photoactive electrode may be exposed to patternedelectromagnetic radiation which may allow for the formation of thecomposition in a pattern. Various techniques may be employed, forexample, passing electromagnetic radiation through a mask (e.g.,lithographical techniques). For example, as shown in FIG. 5D,photoactive electrode 2 is exposed to electromagnetic radiation throughmask 16 such that selected areas of the photoactive electrode areexposed to electromagnetic radiation. The areas of the photoactiveelectrode that were exposed to light comprise composition 8. Theboundaries between areas which comprise the composition and areas whichdo not comprise the composition may be sharp (e.g., the thickness of acomposition in an area is substantially uniform throughout) or gradual(e.g., the thickness of the composition in an area is not substantiallyuniform and/or the thickness of the composition decreases away from thecenter of the area).

Electromagnetic radiation (e.g., in the formation of the compositionassociated with the photoactive electrode or during operation of adevice as described herein) may be provided by any suitable source. Forexample, electromagnetic radiation may be provided by sunlight and/or anartificial light source. In an exemplary embodiment, the electromagneticradiation is provided by sunlight. In some embodiments, light may beprovided by sunlight at certain times of operation of a device (e.g.,during daytime, on sunny days, etc.) and artificial light may be used atother times of operation of the device (e.g., during nighttime, oncloudy days, etc.). Non-limiting examples of artificial light sourcesinclude a lamp (mercury-arc lamp, a xenon-arc lamp, a quartz tungstenfilament lamp, etc.), a laser (e.g., argon ion), and/or a solarsimulator. The spectra of the artificial light source may besubstantially similar or substantially different than the spectra ofnatural sunlight. The light provided may be infrared (wavelengthsbetween about 1 mm and about 750 nm), visible (wavelengths between about380 nm and about 750 nm), and/or ultraviolet (wavelengths between about10 nm and about 380 nm). In some cases, the electromagnetic radiationmay be provided at a specific wavelength, or specific ranges ofwavelengths, for example, through use of a monochromatic light source orthrough the use of filters. The power of the electromagnetic radiationmay also be varied. For example, the light source provided may have apower of at least about 100 W, at least about 200 W, at least about 300W, at least about 500 W, at least about 1000 W, or greater. Theformation and properties of the composition are described herein.

In some cases, the photoactive electrode associated with a catalyticmaterial as described herein may comprise a photoactive composition,such as an n-type semiconductor. The photoactive composition may beselected such that the band gap is between about 1.0 and about 2.0 eV,between about 1.2 and about 1.8 eV, between about 1.4 and about 1.8 eV,between about 1.5 and about 1.7 eV, is about 2.0 eV, or the like. Thephotoactive composition may also have a Fermi level which is compatiblewith the electrolyte and/or a small work function (e.g., such thatelectrons may diffuse into the water to attain thermal equilibrium).This may cause the energy bands of the photoactive composition to bendup near the interface of the electrolyte. In some cases, the photoactiveelectrode may be transparent, substantially transparent, substantiallyopaque, or opaque. In an exemplary embodiment, the photoactive electrodeand the composition associated with the photoactive electrode aretransparent and/or substantially transparent. The photoactive electrodemay be a solid, semi-porous or porous. Non-limiting examples ofphotoactive compositions (or, in some cases, n-type semiconductingmaterials) include TiO₂, WO₃, SrTiO₃, TiO₂—Si, BaTiO₃, LaCrO₃—TiO₂,LaCrO₃—RuO₂, TiO₂—In₂O₃, GaAs, GaP, p-GaAs/n-GaAs/pGa_(0.2)In_(0.48)P,AlGaAs/SiRuO₂, PbO, FeTiO₃, KTaO₃, MnTiO₃, SnO₂, Bi₂O₃, Fe₂O₃ (includinghematite), ZnO, CdS, MoS₂, CdTe, CdSe, CdZnTe, ZnTe, HgTe, HgZnTe, HgSe,ZnTe, ZnS, HgCdTe, HgZnSe, etc., or composites thereof. In some cases,the photoactive composition may be doped. For example, TiO₂ may be dopedwith Y, V, Mo, Cr, Cu, Al, Ta, B, Ru, Mn, Fe, Li, Nb, In, Pb, Ge, C, N,S, etc., and SrTiO₃ may be doped with Zr. The photoactive compositionmay be provided in any suitable morphology or arrangement, for example,including single crystal wafers, coatings (e.g., thin films),nanostructured arrays, nanowires, etc. Those of ordinary skill in theart will be aware of methods and techniques for preparing a photoactivecomposition in a chosen form. For example, doped TiO₂ may be prepared bysputtering, sol-gel, and/or anodization of Ti.

In an exemplary embodiment, the photoactive composition may comprisealpha-Fe₂O₃, also known as hematite. In some embodiments, hematite maybe doped, for example, with Nb, Si, or In. Hematite has a band gap ofabout 2 eV and in some cases, has been found to absorb about 40% of thesolar flux at ground level. Hematite may be provided in any suitablearrangement, for example, as a single crystal, as a coating (e.g., film)on a surface of a material (e.g., SnO₂ glass, Ti, etc.), as nanowires(e.g., on a material), etc.

In some cases, the photoactive electrode may consist essentially of thephotoactive composition (e.g., the photoactive composition forms thephotoactive electrode). In such cases, the photoactive composition maybe a single crystal or polycrystalline. The photoactive composition mayor may not comprise interfaces (e.g., grain boundaries, surface lineardefects, etc.). In some cases, the macroscale (e.g., propertiesrepresentative of the composition as a continuum such as electronicstructure, Fermi energy, etc.) and microscale properties (e.g.,properties of specific sites on the surface such as surface-activecenters formed by surface defects) of the photoactive composition may befound to effect the reactivity and photoreactivity of the photoactivecomposition.

In other cases, the photoactive electrode may not consist essentially ofthe photoactive composition. For example, the photoactive electrode maycomprise the photoactive composition and a second material. The secondmaterial, in some embodiments, may form a core and the photoactivecomposition may substantially cover the core. In other embodiments, thephotoactive composition may be formed on only a portion of the secondmaterial (e.g., one side of the material). In some embodiments, thesecond material may be non-conductive (e.g.,) inorganic substrates,(e.g., quartz, glass, etc.) and polymeric substrates (e.g., polyethyleneterephthalate, polyethylene naphthalate, polycarbonate, polystyrene,polypropylene, etc.) or conductive (e.g., metal, metal oxides, etc.).For example, the photoactive composition may be formed on the surface(e.g., as a film, as particles, as nanotubes) of a material (e.g., Ti,stainless steel, fluorine-doped SnO₂ coated glass (e.g., FTO), etc.).The photoactive composition may be formed on a second material usingtechniques known to those of ordinary skill in the art (e.g., solutiontechniques, sputtering, ultrasonic spray coating, chemical vapordeposition, etc.). The thickness of the photoactive composition may beat least about 10 nm, at least about 100 nm, at least about 1 um, atleast about 10 um, at least about 100 um, at least about 1 mm, orgreater. Methods for determining the thickness of a material isdescribed herein.

In some cases, a photoactive electrode may comprise a photoactivecomposition and a photosensitizing agent. For example, the photoactivecomposition may be associated with a photosensitizing agent (e.g., anorganic dye). The photosensitizing agent may increase the conversionefficiency of the reactions. As an illustrative embodiment,electromagnetic radiation absorbed by the dye causes dye molecules to betransferred from a ground-state (Dye) to an excited state (Dye*) (e.g.,see Equation 8). The excited state dyes may transfer electrons to thephotoactive composition, resulting in the formation of a higheroxidation state dye (Dye⁺) and a reduced photoactive composition (e)(e.g., see Equation 9). The oxidized dye molecules may react with water,thereby resulting in the formation of oxygen at a photoactive electrode(e.g., see Equation 10). The electrons may be transferred from thephotoactive composition to an electrode (e.g., through a circuit) wherethey may react with protons to produce hydrogen gas (e.g., see Equation11), wherein the photoactive electrode is biased positively with respectto the electrode.

Dye+hv→Dye*  (8)

Dye*→Dye⁺ +e ⁻  (9)

Dye⁺+½H₂O→Dye+¼O₂+H⁺  (10)

H⁺ +e ⁻→½H₂  (11)

FIG. 6 shows an energy diagram for a photoelectrochemical devicecomprising a photoactive electrode, and an electrode, wherein thephotoactive electrode comprises a photoactive composition and a dye,wherein the electrons and electrons holes are transferred as discussedabove.

A wide variety of photosensitizing agents may be applied to and/orassociated with a photoactive composition. In some cases, thephotoactive electrode may consist essentially of the photoactivecomposition and the photosensitizing agent (for example, in instanceswhere the photosensitizing agent is formed on a surface of a photoactivematerial). In other cases, the photoactive electrode may not consistessentially of the photoactive composition and the photosensitizingagent (for example, in instances where the photoactive composition isformed on a substrate (e.g., as a film) and the photosensitizing agentis formed (e.g., as a film) on the photoactive composition film). Thephotosensitizing agent may have a single, a narrow range (e.g., lessthan about 100 nm range), a plurality, and/or a wide range (e.g.,greater than about 100 nm range) of light absorption peaks. In somecases, the absorption may occur at a wavelength(s) between about 300 nmand about 1000 nm. In some cases, the photosensitizing agent maycomprise a metal complex dye, an organic dye, quantum dots, etc. Quantumdots will be known to those of ordinary skill in the art and maycomprise ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, Bi₂S₃, HgS, HgSe, HgTe,MgTe, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlAs, AlP, AlSb, AlS,and the like, or combinations thereof (e.g., CdTe/CdSe(core/shell),CdSe/ZnTe(core/shell)). Quantum dots may allow for improved stability ascompared to some metal or organic dyes, tailoring of the band gap of thequantum dots (e.g., by size quantification), and/or tailoring of theoptical absorption of the quantum dots.

In some cases a metal complex dye may comprise a metal such asruthenium, platinum, or any other suitable metal and an organiccomponent (e.g., a ligand) such as biquinoline, bipyridyl,phenanthroline, thiocyanic acid or derivatives thereof. In someinstances, an organic dye may comprise an organic component such as aporphyrin-based system. The organic dyes may or might not comprise atleast one metal (e.g., Zn, Mg, etc.). In some cases, the sensitizingagent may comprise a composition of the formula ML_(x)(L′)_(y)(SCN)_(z)where M is a metal (e.g., Ru), L and L′ may be the same or different andare polypyridyl ligands (e.g., 4,4″-(CO₂H)-2,2″-bipyridine), and x, y,and z can be the same or different and are any whole number 0, 1, 2, 3,etc.

In some cases, the photosensitizing agent comprises a porphyrin-basedsystem, for example:

wherein R¹, R², R³, and R⁴ can be the same or different and are H, analkyl, an alkenyl, an alkynyl, a heteroalkyl (e.g., CF₂CF₂CF₃), aheteroalkenyl, a heteroalkynyl, an aryl, or a heteroaryl, all optionallysubstituted, or are optionally absent (e.g., such that the compound isan anion, dianion, etc.). In some cases, additional carbons on theporphyrin may be optionally substituted. In some instances, theporphyrin may be an anion, dianion, etc. (e.g., such that at least onecenter nitrogen atom is an anion). In some embodiments, theporphyrin-based system may comprise a metal ion (e.g., such that theporphyrin is an anion or a dianion, etc., and the metal ion iscoordinated in the center of the porphyrin by the nitrogen atoms).Non-limiting examples of such metals include Ru, Rh, Fe, Co, Mg, Al, Ag,Au, Zn, Sn, etc., as known to those of ordinary skill in the art. In aparticular case, at least one of R¹ through R⁴ is an aryl, for example,—C₆H₅, —C₆F₅, —C₆H₄(COOH), —C₆H₄OH, —C₆H₄(CH₃), —C₆H₄(C(═O)OCH₃),(ortho, meta, or para)-C₆H₃X₂ where X is a halide (e.g., F, Cl, Br, I),etc. Non-limiting examples of porphyrins include, but are not limitedto:

Additional suitable photosensitizing agents may include, for example,dyes that include functional groups, such as carboxyl and/or hydroxylgroups that can chelate to the nanoparticles, e.g., to Ti(IV) sites on aTiO₂ surface. Examples of suitable dyes include, but are not limited to,anthocyanins, phthalocyanines, merocyanines, cyanines, squarates,eosins, and metal-containing dyes. In some cases, a metal-containing dyemay be a polypyridyl complex of ruthenium(II) (e.g.,cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II),tris(isothiocyanato)-ruthenium(II)-2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylicacid,cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium,cis-bis(isocyanato)(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium (II),and tris(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) dichloride).

The porosity of a photoactive electrode (or other component, forexample, a photoanode) may be measured as a percentage or fraction ofthe void spaces in the photoactive electrode. The percent porosity of aphotoactive electrode may be measured using techniques known to those ofordinary skill in the art, for example, using volume/density methods,water saturation methods, water evaporation methods, mercury intrusionporosimetry methods, and nitrogen gas adsorption methods. In someembodiments, the photoactive electrode may be at least about 10% porous,at least about 20% porous, at least about 30% porous, at least about 40%porous, at least about 50% porous, at least about 60% porous, orgreater. The pores may be open pores (e.g., have at least one part ofthe pore open to an outer surface of the electrode and/or another pore)and/or closed pores (e.g., the pore does not comprise an opening to anouter surface of the electrode or another pore). In some cases, thepores of a photoactive electrode may consist essentially of open pores(e.g., the pores of the photoactive electrode are greater than at least70%, greater than at least 80%, greater than at least 90%, greater thanat least 95%, or greater, of the pores are open pores). In some cases,only a portion of the photoactive electrode may be substantially porous.For example, in some cases, only a single surface of the photoactiveelectrode may be substantially porous. As another example, in somecases, the outer surface of the photoactive electrode may besubstantially porous and the inner core of the photoactive electrode maybe substantially non-porous. In a particular embodiment, the entirephotoactive electrode is substantially porous.

In some embodiments, the photoactive electrode may have a high surfacearea (e.g., geometric or total surface area). In some cases, the surfacearea of the photoactive electrode may be greater than about 0.01 m²/g,greater than about 0.05 m²/g, greater than about 0.1 m²/g, greater thanabout 0.5 m²/g, greater than about 1 m²/g, greater than about m²/g,greater than about 10 m²/g, greater than about 20 m²/g, greater thanabout 30 m²/g, greater than about 50 m²/g, greater than about 100 m²/g,greater than about 150 m²/g, greater than about 200 m²/g, greater thanabout 250 m²/g, greater than about 300 m²/g, or the like. In othercases, the surface area of the photoactive electrode may be betweenabout 0.01 m²/g and about 300 m²/g, between about 0.1 m²/g and about 300m²/g, between about 1 m²/g and about 300 m²/g, between about 10 m²/g andabout 300 m²/g between about 0.1 m²/g and about 250 m²/g, between about50 m²/g and about 250 m²/g, or the like. In some cases, the surface areaof the photoactive electrode may be due to the photoactive electrodecomprising a highly porous material. The surface area of a photoactiveelectrode may be measured using various techniques, for example, opticaltechniques (e.g., optical profiling, light scattering, etc.), electronbeam techniques, mechanical techniques (e.g., atomic force microscopy,surface profiling, etc.), electrochemical techniques (e.g., cyclicvoltammetry, etc.), etc., as known to those of ordinary skill in theart.

The photoactive electrode may be of any size or shape. Non-limitingexamples of shapes include sheets, cubes, cylinders, hollow tubes,spheres, and the like. The photoactive electrode may be of any size,provided that at least a portion of the photoactive electrode may beimmersed in the solution comprising the metal ionic species and theanionic species. The methods described herein are particularly amenableto forming the catalytic material on any shape and/or size ofphotoactive electrode. In some cases, the maximum dimension of thephotoactive electrode in one dimension may be at least about 1 mm, atleast about 1 cm, at least about 5 cm, at least abut 10 cm, at leastabout 1 m, at least about 2 m, or greater. In some cases, the minimumdimension of the photoactive electrode in one dimension may be less thanabout 50 cm, less than about 10 cm, less than about 5 cm, less thanabout 1 cm, less than about 10 mm, less than about 1 mm, less than about1 um, less than about 100 nm, less than about 10 nm, less than about 1nm, or less. Additionally, the photoactive electrode may comprise ameans to connect the photoactive electrode to a power source and/orother electrical devices. In some cases, the photoactive electrode maybe at least about 10%, at least about 30%, at least about 50%, at leastabout 60%, at least about 70%, at least about 80%, at least about 90%,at least about 95%, or at least about 100% immersed in the solution.

The photoactive electrode may or may not be substantially planar. Forexample, the photoactive electrode may comprise ripples, waves,dendrimers, spheres (e.g., nanospheres), rods (e.g., nanorods), apowder, a precipitate, a plurality of particles, and the like. In someembodiments, the surface of the photoactive electrode may be undulating,wherein the distance between the undulations and/or the height of theundulations are on a scale of nanometers, micrometers, millimeters,centimeters, or the like. In some instances, the planarity of thephotoactive electrode may be determined by determining the roughness ofthe photoactive electrode. As used herein, the term “roughness” refersto a measure of the texture of a surface (e.g., photoactive electrode),as will be known to those of ordinary skill in the art. The roughness ofthe photoactive electrode may be quantified, for example, by determiningthe vertical deviations of the surface of the photoactive electrode fromplanarity. Roughness may be measured using contact (e.g., dragging ameasurement stylus across the surface such as a profilometers) ornon-contact methods (e.g., interferometry, confocal microscopy,electrical capacitance, electron microscopy, etc.). In some cases, thesurface roughness, R_(a), may be determined, wherein R_(a) is thearithmetic average deviations of the surface valleys and peaks,expressed in micrometers. The R_(a) of a non-planar surface may begreater than about 0.1 um, greater than about 1 um, greater than about 5um, greater than about 10 um, greater than about 50 um, greater thanabout 100 um, greater than about 500 um, greater than about 1000 um, orthe like.

Without wishing to be bound by theory, the formation of a catalyticmaterial on a photoactive electrode may proceed according to thefollowing example. A photoactive electrode may be immersed in a solutioncomprising metal ionic species (M) with an oxidation state of (n) (e.g.,M^(n)) and anionic species (e.g., A^(−y)). As voltage is applied to thephotoactive electrode, metal ionic species near to the photoactiveelectrode may be oxidized to an oxidation state of (n+x) (e.g.,M^((n+x))). The oxidized metal ionic species may interact with ananionic species near the electrode to form a substantially insolublecomplex, thereby forming a catalytic material. In some cases, thecatalytic material may be in electrical communication with thephotoactive electrode. A non-limiting example of this process isdepicted in FIG. 7. FIG. 7A shows a single metal ionic species 40 withan oxidation state of (n) in solution 42. Metal ionic species 44 may benear photoactive electrode 46, as depicted in FIG. 7B. As shown in FIG.7C, metal ionic species may be oxidized to an oxidized metal ionicspecies 48 with an oxidation state of (n+x) and (x) electrons 50 may betransferred to photoactive electrode 52 or to another species near orassociated with the metal ionic species and/or the photoactiveelectrode. FIG. 7D depicts a single anionic species 54 nearing oxidizedmetal ionic species 56. In some instances, as depicted in FIG. 7E,anionic species 58 and oxidized metal ionic species 60 may associatewith photoactive electrode 62 to form a catalytic material. In someinstances, the oxidized metal ionic species and the anionic species mayinteract and form a complex (e.g., a salt) before associating with theelectrode. In other instances, the metal ionic species and anionicspecies may associate with each other prior to oxidation of the metalionic species. In other instances, the oxidized metal ionic speciesand/or anionic species may associate directly with the photoactiveelectrode and/or with another species already associated with thephotoactive electrode. In these instances, the metal ionic speciesand/or anionic species may associate with the photoactive electrode(either directly, or via formation of a complex) to form the catalyticmaterial (e.g., a composition associated with the photoactiveelectrode).

In some embodiments, a photoanode may be formed by immersing aphotoactive electrode associated with a material comprising a metalionic species (e.g., a layer of a metal such as metallic cobaltassociated with a photoactive electrode) in a solution comprising ionicspecies (e.g., phosphate). The metal ionic species (e.g., in anoxidation state of M^(n)) may be oxidized and/or dissociated from thephotoactive electrode into solution. The metal ionic species that areoxidized and/or dissociated from the photoactive electrode may interactwith anionic species and/or other species, and may re-associate with thephotoactive electrode, thereby re-forming a catalytic material.

As noted above, one aspect of the invention involves an efficient androbust catalytic material (for electrolysis of water and/or otherelectrochemical reactions) that is primarily photoactiveelectrode-associated, rather than functioning largely as a homogeneoussolution-based catalytic materials. Such a catalytic material“associated with” a photoactive electrode will now be described withreference to a metal ionic species and/or anionic species which candefine a catalytic material of the invention. In some cases, the anionicspecies and the metal ionic species may interact with each other priorto, simultaneously to, and/or after the association of the species withthe photoactive electrode, and result in a catalytic material with ahigh degree of solid content resident on, or otherwise immobilized withrespect to, the photoactive electrode. In this arrangement, thecatalytic material can be solid including various degrees of electrolyteor solution (e.g., the material can be hydrated with various amounts ofwater), and/or other species, fillers, or the like, but a unifyingfeature among such catalytic material associated with photoactiveelectrodes is that they can be observed, visually or through othertechniques described more fully below, as largely resident on orimmobilized with respect to the photoactive electrode, either inelectrolyte solution or after removal of the photoactive electrode fromsolution.

In some cases, the catalytic material may associate with the photoactiveelectrode via formation of a bond, such as an ionic bond, a covalentbond (e.g., carbon-carbon, carbon-oxygen, oxygen-silicon, sulfur-sulfur,phosphorus-nitrogen, carbon-nitrogen, metal-oxygen, or other covalentbonds), a hydrogen bond (e.g., between hydroxyl, amine, carboxyl, thiol,and/or similar functional groups), a dative bond (e.g., complexation orchelation between metal ions and monodentate or multidentate ligands),Van der Waals interactions, and the like. “Association” of thecomposition (e.g., catalytic material) with the photoactive electrodewould be understood by those of ordinary skill in the art based on thisdescription. In some embodiments, the interaction between a metal ionicspecies and an anionic species may comprise an ionic interaction,wherein the metal ionic species is directly bound to other species andthe anionic species is a counterion not directly bound to the metalionic species. In a specific embodiment, an anionic species and a metalionic species form an ionic bond and the complex formed is a salt.

A catalytic material associated with a photoactive electrode may be mostoften arranged with respect to the photoactive electrode so that it isin sufficient electrical communication with the photoactive electrode tocarry out purposes of the invention as described herein. “Electricalcommunication,” as used herein, is given its ordinary meaning as wouldbe understood by those of ordinary skill in the art whereby electronscan flow between the photoactive electrode and the catalytic material ina facile enough manner for the photoanode to operate as describedherein. That is, charge may be transferred between the photoactiveelectrode and the catalytic material (e.g., the metal ionic speciesand/or anionic species present in the catalytic material). In onearrangement, the composition is in direct contact with the photoactiveelectrode. In another arrangement, a material may be present between thecomposition and the photoactive electrode (e.g., a photosensitizingagent, an insulator, a conducting material, a semiconducting material,etc.).

In some cases, the composition may be in “direct electricalcommunication” with the photoactive electrode. “Direct electricalcommunication,” as used herein, is given its ordinary meaning as definedabove with respect to electrical communication, but in this instance,the photoactive electrode and the composition are in direct contact withone another (e.g., as opposed to through a secondary material, throughuse of circuitry, etc.). In some embodiments, the composition and thephotoactive electrode may be integrally connected. The term “integrallyconnected,” when referring to two or more objects, means objects that donot become separated from each other during the course of normal use,e.g., separation requires at least the use of tools, and/or by causingdamage to at least one of the components, for example, by breaking,peeling, dissolving, etc. A composition may be considered to be indirect electrical communication with a photoactive composition duringoperation of a photoanode even in instances where a portion of thecomposition may dissociate from the photoactive composition when takingpart in a dynamic equilibrium.

In some embodiments, a composition (e.g., catalytic material) may be in“indirect electrical communication” with a photoactive electrode. Thatis, a material and/or circuitry may be interposed between thecomposition and the photoactive electrode. In some cases, the materialmay be a “hole-tunneling barrier.” That is, a material through whichelectron-holes generated in the photoactive electrode may tunnel throughto access the composition (e.g., catalytic material). The hole-tunnelingbarrier may aid in protecting the photoactive electrode from corrosion.In some instances, the material may be a conducting material, therebyallowing electrons to flow between the photoactive electrode and thecomposition. The electrons may be used for the production of oxygen gasfrom water via the composition. Without wishing to be bound by theory, amaterial disposed between the composition and the photoactive electrodemay act as a membrane and may allow for the transmission of electronholes generated at the photoactive electrode to the composition. Thisarrangement may be advantageous in devices where the separation ofoxygen gas and hydrogen gas formed from the oxidation of water isimportant. The presence of the material may prevent oxygen gas formedwhere the composition is present from transversing the device andentering the area where hydrogen gas is produced. In some cases, thematerial may be selected such that no oxygen gas is produced in thematerial (e.g., in instances where the overpotential for production ofoxygen gas is high).

In instances where the photoanode comprises a catalytic material and aphotoactive electrode (e.g., comprising a photoactive composition and aphotosensitizing agent), the catalytic material, the photoactivecomposition and the photosensitizing agent may be in electricalcommunication with one another. In some cases, the photoactivecomposition and the photosensitizing agent and/or the photosensitizingagent and the catalytic material may be in direct electricalcommunication with one another and/or integrally connected. For example,in some cases, a photoanode may comprise a photoactive composition indirect electrical communication with a photosensitizing agent, whereinthe photosensitizing agent is in direct electrical communication with acatalytic material (e.g., the photoactive composition comprises acoating of a photosensitizing agent followed by a coating of catalyticmaterial).

It should be understood that while much of the discussion herein focuseson a photoanode comprising a catalytic material associated with aphotoactive electrode (e.g., the photoactive electrode and the catalyticmaterial are in direct electrical communication), this is by no meanslimiting, and the photoanode may comprise one or more materials betweenthe photoactive electrode and the catalytic material (e.g., such thatthe photoactive electrode and the catalytic material are in indirectelectrical communication).

One aspect of the invention involves the development of a regenerativecatalytic photoanode. As used herein, a “regenerative photoanode” refersto a photoanode which is capable of being compositionally regenerated asit is used in a catalytic process, and/or over the course of a changebetween catalytic use settings. Thus, a regenerative photoanode of theinvention is one that includes one or more species associated with thephotoanode (e.g., adsorbed on the photoanode) which, under certainconditions, dissociate from the photoanode, and then a significantportion or substantially all of those species re-associate with thephotoanode at a later point in the photoanode's life or use cycle. Forexample, at least a portion of the catalytic material may dissociatefrom the photoanode and become solvated or suspended in a fluid to whichthe photoanode is exposed, and then become re-associated (e.g.,adsorbed) at the photoanode. The disassociation/re-association may takeplace as a part of the catalytic process itself, as catalytic speciescycle between various states (e.g., oxidation states), in which they aremore or less soluble in the fluid. This phenomenon during use, forexample, nearly or essentially steady-state use of the electrode, can bedefined as a dynamic equilibrium. “Dynamic equilibrium,” as used herein,refers to an equilibrium comprising metal ionic species and anionicspecies, wherein at least a portion of the metal ionic species arecyclically oxidized and reduced (as discussed elsewhere herein).Regeneration over the course of a change between catalytic use settingscan be defined by a dynamic equilibrium which experiences a significantdelay in its cyclical nature.

In some embodiments, at least a portion of the catalytic material maydissociate from the photoanode and become solvated or suspended in thefluid (or solution and/or to other medium) as a result of a significantreaction setting change, and then become re-associated at a later stage.A significant reaction setting change, in this context, can be asignificant change in potential applied to the electrode, significantlydifferent current density at the photoanode, significantly differentproperties of a fluid to which the photoanode is exposed (or removaland/or changing of the fluid), or the like. In one embodiment, thephotoanode is exposed to catalytic conditions under which the catalyticmaterial catalyzes a reaction, then the circuit of which the photoanodeis a part is changed so that the catalytic reaction is significantlyslowed or even essentially stopped (e.g., the process is turned off),and then the system can be returned to the original catalytic conditions(or similar conditions that promote the catalysis), and at least aportion, or essentially all of the catalytic material, can re-associatewith the photoanode. Re-association of some or essentially all of thecatalytic material with the photoanode can occur during use and/or uponchange in conditions as noted above, and/or can occur upon exposure ofthe catalytic material, the electrode, or both to a regenerativestimulus, such as a regenerative electrical potential, current,temperature, electromagnetic radiation, or the like. In some cases, theregeneration may comprise a dynamic equilibrium mechanism involvingoxidation and/or reduction processes, as described elsewhere herein.

Regenerative photoanodes of the invention can exhibit disassociation andre-association of catalytic species at various levels. In one set ofembodiments, at least about 0.1% by weight of catalytic materialassociated with the photoanode disassociates as described herein, and inother embodiments as much as about 0.25%, about 0.5%, about 0.6%, about0.8%, about 1.0%, about 1.25%, about 1.5%, about 1.75%, about 2.0%,about 2.5%, about 3%, about 4%, about 5%, or more of the catalyticmaterial disassociates, and some or all re-associates as discussed. Invarious embodiments, of the amount of material that disassociates, atleast about 50%, at least about 60%, at least about 70%, at least about80%, at least about 85%, at least about 90%, at least about 95%, atleast about 97%, at least about 98%, at least about 99%, or essentiallyall material re-associates. Those of ordinary skill in the art willunderstand the meaning of disassociation and re-association of materialin this regard, and will know of techniques for measuring these factors(for example, scanning electron microscopy and/or elemental analyses ofthe electrode, chemical analysis of the fluid, photoanode performance,or any combination). Further, those of ordinary skill in the art willquickly be able to select catalytic materials which meet theseparameters with knowledge of solubilities and/or catalytic reactionscreening, or combinations. As a specific example, in some cases, duringuse of a catalytic material comprising cobalt ions and anionic speciescomprising phosphorus, at least a portion of the cobalt ions and theanionic species comprising phosphorus periodically associate anddissociate from the electrode.

Catalytic materials of the invention may also exhibit significantrobustness through varying levels of use in a way that is a significantimprovement over the general state of the art. Through a mechanism thatmay be related to regeneration as described herein, systems and/orphotoanodes employing catalytic materials of the invention may beoperated at varying rates of applied energy, as would result from beingdriven by power sources that vary (e.g., wind power, solar power whichgenerally varies over the daily cycle and weather patterns, etc.),and/or go through full on/off energy cycles. In particular, systemsand/or photoanodes of the invention may be cycled such that potentialand/or current supplied to the system and/or photoanode is reduced by atleast about 20%, at least about 40%, at least about 60%, at least about80%, at least about 90%, at least about 95%, or essentially 100% frompeak use current, for at least from a period of about 2 minutes, atleast about 5 minutes, at least about 10 minutes, at least about 20minutes, at least about 30 minutes, at least about 1 hour, at leastabout 2 hours, at least about 3 hours, at least about 5 hours, at leastabout 8 hours, at least about 12 hours, at least about 24 hours orgreater, and cycled at least about five times, at least about 10 times,at least about 20 times, at least about 50 times, or more, while overallperformance (e.g., overpotential at a selected current density,production of oxygen gas, production of water, etc.) of the systemand/or photoanode, decreases by no more than about 20%, no more thanabout 10%, no more than about 8%, no more than about 6%, no more thanabout 4%, no more than about 3%, no more than about 2%, no more thanabout 1%, or the like. In some cases, the performance measurement may betaken at about the same period of time after reapplication of thevoltage/current to the photoanode/system (e.g., after voltage/currenthas been reapplied to the photoanode/system for about 1 minute, about 5minutes, about 10 minutes, about 30 minutes, about 60 minutes, etc.).

It should be understood, however, that not every metal ionic speciesand/or anionic species which exhibits a change in oxidation state candissociate and re-associate with a photoactive electrode. In some cases,only a small portion (e.g., less than about 20%, less than about 15%,less than about 10%, less than about 5%, less than about 2%, less thanabout 1%, or less) of the oxidized/reduced metal ionic species maydissociate/associate with the photoactive electrode during operation orbetween uses.

Those of ordinary skill in the art also will quickly recognize thesignificance of the contribution of this aspect (e.g., regenerationmechanism) of the invention to the field. It is known that degradationof catalytic materials and photoanodes can be problematic during theiruse, or especially when they are shut off between uses, especially inthe case of metal organic, inorganic, and/or organometallic catalyticmaterials exposed to conditions previously assumed necessary forstandard catalytic processes, and/or conditions described in accordancewith catalysis according to the present invention (e.g., metal oxidesand/or hydroxides or other catalytic materials used in processes at highpH). Without wishing to be bound by any theory, the inventors believetheir development of regenerative photoanodes relates to selection ofspecies with high enough stability under catalytic conditions describedherein, and/or combination of this feature with the process of someamount of catalytic material loss from the photoanode followed byre-association of the material with the photoanode, which is believed toinvolve a material cleansing process. The regeneration mechanism mayalso inhibit unwanted coating or other accumulation of auxiliaryspecies, which do not play a role in the catalytic process and which mayinhibit catalysis and/or other performance characteristics.

Regenerative photoanodes of the invention also exhibit strong andsurprising performance associated with their regenerative properties.Thus, in various embodiments, a regenerative photoanode of the inventionnot only has good long-term robustness, but exhibits surprisingly goodstability even upon significant variations in its use. Significant usevariations can involve the photoanode and its corresponding catalysissystem being switched from on to off states, or other significantchanges in use profile. This can be particularly important where thephotoanode is driven by solar power, where variation in the sunintensity can vary dramatically. In such a situation, a photoanode ofthe invention may be operating at essentially full capacity at times,and be switched off at times (e.g., where an electrical circuit in whichthe photoanode exists is in an “open” position). The photoanode of theinvention exhibits robustness such that, when it is operated at or closeto its highest capacity for catalysis, i.e., at its highest rate ofcatalysis, and then switched off (“open circuit”), and this is repeatedat least ten times, the photoanode exhibits less than about 10%, lessthan about 5%, less than about 4%, less than about 3%, less than about2%, less than about 1%, less than about 0.5%, or less than about 0.25%loss in performance. In this case, performance can be measured ascurrent density at a particular set overpotential, with all otherconditions being essentially identical between all tests. Of course, thephotoanode need not necessarily be switched between essentially fullcapacity and off in this way, but a photoanode of the invention, whentreated in this way, will exhibit a level of robustness.

In some cases, the photoanode may be capable of regeneration, asdescribed herein, in a closed system. That is, the photoanode may becapable of regeneration without removal and/or addition of anymaterial(s) that aids and/or assists in the regeneration of thephotoanode. Alternatively, removal of and/or addition of such materialin only small amounts in various embodiments, such as, for example, nomore than about 1% by weight, or no more than about 2%, 4%, 6%, 10%, ormore, by weight of such material. For example, in instances where thephotoanode comprises a regenerative catalytic material, the catalyticmaterial may be capable of regeneration without addition of any of thecomponents comprised in the catalytic material (e.g., metal ionicspecies and/or anionic species where the catalytic material is composedof these materials) in such a closed system, or addition of one or suchcomponents in amounts no more than those described above in variousembodiments. It should be understood, however, that a “closed system” asused herein does not exclude addition or removal of species that do notdefine, or can not react within the system to define, the catalyticmaterial. For example, additional fuel and/or water may be provided tosuch a system.

In some embodiments, a dynamic equilibrium may comprise at least aportion of the metal ionic species being cyclically oxidized andreduced, wherein the metal ionic species are thereby associated anddisassociated, respectively, from the photoactive electrode. An exampleof a dynamic equilibrium (or regenerative mechanism) which can, but neednot necessarily, take place in accordance with the invention is depictedin FIG. 8. FIG. 8A depicts a photoanode comprising photoactive electrode80 and catalytic material 82 comprising metal ionic species 84 andanionic species 86. The dynamic equilibrium is depicted in FIGS. 8B-8C.FIG. 8B shows the same photoanode, wherein a portion of metal ionicspecies 88 and anionic species 90 have disassociated from photoactiveelectrode 92. FIG. 8C shows the same photoanode at some point later intime where a portion of the metal ionic species and anionic species(e.g., 94) which disassociated from the photoactive electrode havere-associated with photoactive electrode 96. Additionally, differentmetal ionic species and anionic species (e.g., 98) may havedisassociated from the photoactive electrode. Metal ionic species andanionic species can repeatedly disassociate and associate with thephotoactive electrode. For example, the same metal ionic species andanionic species may disassociate and associate with the photoactiveelectrode. In other instances, the metal ionic species and/or anionicspecies may only disassociate and/or associate with the photoactiveelectrode once. A single metal ionic species may associate with thephotoactive electrode simultaneously as a second single metal ionicspecies disassociates from the photoanode. The number of single metalionic species and/or single anionic species that may disassociate and/orassociate simultaneously and/or within the lifetime of the photoanodehas no numerical limit.

It should be understood that a solution in which metal ionic speciesand/or anionic species may be solubilized may be transiently present(e.g., the solution might not necessarily be in contact with thephotoactive electrode during the entire operation and/or formation ofthe photoanode). For example, in instances where water is provided tothe photoanode in a gaseous state, in some embodiments, the solution maybe comprised of transiently formed aqueous molecules and/or droplets onthe surface of the photoanode and/or electrolyte. In other instances,where the electrolyte is a solid, the solution may be present inaddition to the electrolyte (e.g., as water droplets on the surface ofthe photoanode and/or solid electrolyte) or in combination with the fuel(e.g., water). The photoanode may be operated with a combination ofsolid electrolyte/gaseous fuel, fluid electrolyte/gaseous fuel, solidelectrolyte/fluid fuel, fluid electrolyte/fluid fuel, or any combinationthereof.

In some embodiments, during the dynamic equilibrium, at least a portionof the metal ionic species are cyclically oxidized and reduced. That is,the oxidation state of at least a portion of the metal ionic speciesinvolved in the dynamic equilibrium is repeatedly changed during thedynamic equilibrium. A change in the oxidation state of a metal ionicspecies may also correlate to the association or dissociation of themetal ionic species with the photoactive electrode.

In some embodiments, the metal ionic species in solution may have anoxidation state of (n), while the metal ionic species associated withthe photoactive electrode may have an oxidation state of (n+x), whereinx is any whole number. The change in oxidation state may facilitate theassociation of the metal ionic species on the photoactive electrode. Itmay also facilitate the oxidation of water to form oxygen gas or otherelectrochemical reactions. The cyclically oxidized and reduced oxidationstates for a single metal ionic species in dynamic equilibrium may beexpressed according to Equation 12:

M^(n)

M^(n+x) +x(e ⁻)  (12)

where M is a metal ionic species, n is the oxidation state of the metalionic species, x is the change in the oxidation state, and x(e⁻) is thenumber of electrons, where x may be any whole number. In some cases, themetal ionic species may be further oxidized and/or reduced, (e.g., themetal ionic species may access oxidation states of M^((n+1)), M^((n+2)),etc.)

An illustrative example of changes in oxidation state that may occur fora single metal ionic species during a dynamic equilibrium is shown inFIG. 9. FIG. 9A depicts a photoactive electrode 100 and a single metalionic species 102 in oxidation state of (n), (e.g., M^(n)). The metalionic species 102 may be oxidized to a metal ionic species 104 with anoxidation state of (n+1) (e.g., M^((n+1))) and/or associate with thephotoactive electrode 106, as shown in FIG. 9B. At this point, the metalionic species (e.g., M^((n+1))) may disassociate from the photoactiveelectrode 106 or may undergo a further change in oxidation state. Insome cases, as shown in FIG. 9C, the metal ionic species may be furtheroxidized to a single metal ionic species 108 with an oxidation state of(n+2), (e.g., M^((n+2))) and may remain associated with the photoactiveelectrode (or may disassociate from the photoactive electrode). At thispoint, metal ionic species 108 (e.g., M^((n+2))) may accept electrons(e.g., from water or another reaction component) and may be reduced toform metal ionic species with a reduced oxidation state of (n) or (n+1)(e.g., M^((n+1)), 106 or M^(n), 102). In other cases, the metal ionicspecies 106 (e.g., M^((n+1))) may be reduced and reform metal ionicspecies in oxidation state (n) (e.g., M^(n), 102). The metal ionicspecies in oxidation state (n) may remain associated with thephotoactive electrode or may disassociate from the photoactive electrode(e.g., dissociate into solution).

Those of ordinary skill in the art will be able to use suitablescreening tests to determine whether a metal ionic species and/oranionic species are in dynamic equilibrium and/or whether a photoactiveelectrode is regenerative. For example, in some cases, the dynamicequilibrium may be determined using radioisotopes of the metal ionicspecies and/or anionic species. In such cases, a photoanode comprising aphotoactive electrode and a catalytic material comprising radioisotopesmay be prepared. The photoanode may be placed in an electrolyte whichcomprises non-radioactive ionic species. The catalytic material maydissociate from the photoactive electrode and therefore, the solutionmay comprise radioactive isotopes of the anionic species and/or metalionic species. This may be determined by analyzing an aliquot of theelectrolyte for the radioisotopes. Upon application of the voltage tothe photoactive electrode, in instances where the metal ionic speciesand anionic species are in dynamic equilibrium, the radioisotopes of themetal ionic species may re-associate with the photoactive electrode.Aliquots of the electrolyte may be analyzed to determine the amount ofradioisotope present in the electrolyte at various time points afterapplication of the voltage. If the metal ionic species and anionicspecies are in dynamic equilibrium, the percentage of radioisotopes insolution may decrease with time as the radioisotopes re-associate withthe photoactive electrode. This screening technique may be used both todetermine how a catalytic material may be functioning, and to selectmaterials which can be used as catalytic materials suitable for theinvention.

Further techniques useful for selecting suitable catalytic materialfollow. Without wishing to be bound by theory, the solubility of amaterial comprising anionic species and oxidized metal ionic species mayinfluence the association of the metal ionic species and/or anionicspecies with the photoactive electrode. For example, if a materialformed by (c) number of anionic species and (b) number of oxidized metalionic species is substantially insoluble in the solution, the materialmay be influenced to associate with the photoactive electrode. Thisnon-limiting example may be expressed according to Equation 13:

b(M^((n+x)))+c(A^(−y))

{[M]_(b)[A]_(c)}^((b(n+x)−c(y)))(s)  (13)

where M^((n+x)) is the oxidized metal ionic species, A^(−y) is theanionic species, and {[M]_(b)[A]_(c)}^((b(n+x)−c(y))) is at least aportion of catalytic material formed, where b and c are the number ofmetal ionic species and anionic species, respectively. Therefore, theequilibrium may be driven towards the formation of the catalyticmaterial by the presence of an increased amount of anionic species. Insome cases, the solution surrounding the photoactive electrode maycomprise an excess of anionic species, as described herein, to drive theequilibrium towards the formation of the catalytic material associatedwith the photoactive electrode. It should be understood, however, thatthe catalytic material does not necessarily consist essentially of amaterial defined by the formula {[M]_(b)[A]_(c}) ^(n+x−y)), as, in mostcases, additional components can be present in the catalytic material(e.g., a second type of anionic species). However, the guidelinesdescribed herein (e.g., regarding K_(sp)) provide information to selectcomplimentary anionic species and metal ionic species that may aid inthe formation and/or stabilization of the catalytic material. In somecases, the catalytic material may comprise at least one bond between ametal ionic species and an anionic species (e.g., a bond between acobalt ion and an anionic species comprising phosphorus).

Selection of metal ionic species and anionic species for use in theinvention will now be described in greater detail. It is to beunderstood that any of a wide variety of such species meeting thecriteria described herein can be used and, so long as they participatein catalytic reactions described herein, they need not necessarilybehave, in terms of their oxidation/reduction reactions, cyclicalassociation/disassociation from the photoactive electrode etc., in themanner described in the application. But in many cases, metal ionic andanionic species selected as described herein, do behave according to oneor more of the oxidations/reduction and solubility theories describedherein. In some embodiments, the metal ionic species (M^(n)) and theanionic species (A^(−y)) may be selected such that they exhibit thefollowing properties. In most cases, the metal ionic species and theanionic species are soluble in an aqueous solution. In addition, themetal ionic species may be provided in an oxidized form, for examplewith an oxidation state of (n), where (n) is one, two, three, orgreater, i.e., in some cases, the metal ionic species have access to anoxidation state greater than (n), for example, (n+1) and/or (n+2).

The solubility product constant, K_(sp), as will be known to those ofordinary skill in the art, is a simplified equilibrium constant definedfor the equilibria between a composition comprising the species andtheir respective ions in solution and may be defined according toEquation 15, based on the equilibrium shown in Equation 14.

{M_(y)A_(n)}_((s))

py(M)^(n) _((aq))+pn(A)^(−y) _((aq))  (14)

K_(sp)=[M]^(y)[A]^(n)  (15)

In Equations 14 and 15, M is the positively charged metal ionic speciesand A is the anionic species and y is not equal to n. In embodimentswhere y is equal to n, the equation may be simplified as shown inEquation 16.

{M_(y)A_(n)}_((s))

(M)^(n) _((aq))+(A)^(−y) _((aq))  (16)

The solid complex M_(y)A_(n) may disassociate into solubilized metalionic species and anionic species. Equation 15 shows the solubilityproduct constant expression. As will be known to those of ordinary skillin the art, the solubility product constant value may change dependingon the temperature of the aqueous solution. Therefore, when choosingmetal ionic species and anionic species for the formation of aphotoanode, the solubility product constant should be determined at thetemperature at which the photoanode is to be formed and/or operated in.In addition, the solubility of a solid complex may change depending onthe pH. This effect should be taken into account when applying thesolubility product constant to the selection of a metal ionic speciesand an anionic species.

In many cases, the metal ionic species and anionic species are selectedtogether, for example, such that a composition comprising the metalionic species with an oxidation state of (n) and the anionic species issoluble in an aqueous solution, the composition having a solubilityproduct constant which is greater than the solubility product constantof a composition comprising the metal ionic species with an oxidationstate of (n+x) and the anionic species. That is, the compositioncomprising the metal ionic species with an oxidation state of (n) andthe anionic species may have a K_(sp) value substantially greater thanthe K_(sp) for the composition comprising the metal ionic species withan oxidation state of (n+x) and the anionic species. For example, themetal ionic species and anionic species may be selected such that theK_(sp) value of a composition comprising the anionic species and themetal ionic species with an oxidation state of (n) (e.g., M^(n)) isgreater than the K_(sp) value of the composition comprising the anionicspecies and the metal ionic species with an oxidation state of (n+x)(e.g., M^((n+x))) by a factor of at least about 10, at least about 10²,at least about 10³, at least about 10⁴, at least about 10⁵, at leastabout 10⁶, at least about 10⁸, at least about 10¹⁰, at least about 10¹⁵,at least about 10²⁰, at least about 10³⁰, at least about 10⁴⁰, at leastabout 10⁵⁰, and the like. Where these K_(sp) values are realized, acatalytic material may be more likely to serve as a photoanode orphotoactive electrode-associated material.

In some instances, a catalytic material, such as a compositioncomprising a metal ionic species with an oxidation state of (n+x) and ananionic species may have a K_(sp) between about 10⁻³ and about 10⁻⁵⁰. Insome cases, the solubility constant of this composition may be betweenabout 10⁻⁴ and about 10⁻⁵⁰, between about 10⁻⁵ and about 10⁻⁴⁰, betweenabout 10⁻⁶ and about 10⁻³⁰, between about 10⁻³ and about 10⁻³⁰, betweenabout 10⁻³ and about 10⁻²⁰, and the like. In some cases, the solubilityconstant may be less than about 10⁻³, less than about 10⁻⁴, less thanabout 10⁻⁶, less than about 10⁻⁸, less than about 10⁻¹⁰, less than about10⁻¹⁵, less than about 10⁻²⁰, less than about 10⁻²⁵, less than about10⁻³⁰, less than about 10⁻⁴⁰, less than about 10⁻⁵⁰, and the like. Insome cases, the composition comprising metal ionic species with anoxidation state of (n) and the anionic species may have a solubilityproduct constant greater than about 10⁻³, greater than about 10⁻⁴,greater than about 10⁻⁵, greater than about 10⁻⁶, greater than about10⁻⁸, greater than about 10⁻¹², greater than about 10⁻¹⁵, greater thanabout 10⁻¹⁸, greater than about 10⁻²⁰, and the like. In a particularembodiment, the composition comprising metal ionic species and theanionic species may be selected such that the composition comprising themetal ionic species with an oxidation state of (n) and the anionicspecies have a K_(sp) value between about 10⁻³ and about 10⁻¹⁰ and thecomposition comprising the metal ionic species with an oxidation stateof (n+x) and the anionic species have a K_(sp) value less than 10⁻¹⁰.Non-limiting examples of metal ionic species and anionic species thatcan be soluble in an aqueous solution and have a K_(sp) value in asuitable range includes Co(II)/HPO₄ ⁻², Co(II)/H₂BO₃ ⁻, Co(II)/HAsO₄ ⁻²,Fe(II)/CO₃ ⁻², Mn(II)/CO₃ ⁻², and Ni(II)/H₂BO₃ ⁻. In some cases, thesecombinations may additionally comprise at least a second type of anionicspecies, for example, oxide and/or hydroxide ions. The composition thatforms on the photoactive electrode may comprise the metal ionic speciesand anionic species selected, as well as additional components (e.g.,oxygen, water, hydroxide, counter cations, counter anions, etc.).

As noted, a photoanode can be formed by deposition of a catalyticmaterial from solution. Whether the photoanode has been properly formed,with proper association of the catalytic material with the photoactiveelectrode, may be important to monitor, both for selecting proper metalionic species and/or anionic species and, of course, determining whetheran appropriate photoanode has been formed. The photoanode may bedetermined to have been formed using various procedures. In someinstances, the formation of a catalytic material on the photoactiveelectrode may be observed. The formation of the material may be observedby a human eye, or with use of magnifying devices such as a microscopeor via other instrumentation. In one case, application of a voltage tothe photoanode, in conjunction with an appropriate counter electrode (orphotocathode) and other components (e.g., circuitry, power source,electrolyte) may be carried out to determine whether the system producesoxygen gas at the photoanode when the photoanode is exposed to water.

In some cases, the onset potential (and/or minimum overpotential) thatis required by the photoanode to produce oxygen gas may be differentthan the onset potential (and/or overpotential) required by thephotoactive electrode alone. The term, “onset potential,” as usedherein, refers to the potential at which the photocurrent of thephotoanode becomes positive as the potential applied to the photoanodeis swept from negative to positive values. In some cases, the onsetpotential required for the photoanode is less positive than the onsetpotential required for the photoactive electrode alone (i.e., the onsetpotential is less positive for the photoanode that includes both thephotoactive electrode and catalytic material, than for the photoactiveelectrode alone). In some embodiments, the onset potential of aphotoanode comprising a photoactive electrode and a catalytic materialis at least about 100 mV, at least about 200 mV, at least about 250 mV,at least about 300 mV, at least about 350 mV, at least about 400 mV, atleast about 450 mV, at least about 500 mV, or more, less positive thanthe onset potential of the photoactive electrode alone. Or, in somecases, the onset potential is about 100 mV, about 200 mV, about 250 mV,about 300 mV, about 350 mV, about 400 mV, about 450 mV, about 500 mVless positive.

In some cases, the incident photon-to-current conversion efficiency (orIPCE, also known as energy quantum efficiency) that is required by thephotoanode to produce oxygen gas may be different than the IPCE requiredby the photoactive electrode alone. The term “incident photon-to-currentconversion efficiency,” as used herein, refers to a measure of thephoton to electron conversion efficiency at a specific wavelength. Aswill be known to those of ordinary skill in the art, IPCE may bedetermined from measuring the monochromatic light power density, and maybe calculated as a function of short circuit current density, incidentlight power density, and wavelength. In some cases, the IPCE for thephotoanode is greater than the IPCE for the photoactive electrode alone.In some embodiments, the IPCE of a photoanode comprising a photoactiveelectrode and a catalytic material is about 1%, about 2%, about 5%,about 10%, about 20%, about 25%, about 30%, about 40%, about 50%, about75%, about 100%, or more, greater than the IPCE of the photoactiveelectrode alone. In some cases, the IPCE is measured with solarsimulated light (e.g., AM-1.5 illumination).

In some cases, a device (e.g., photoelectrochemical cell) comprising thephotoanode may be characterized by its overall efficiency for conversionof solar energy to chemical energy. In such embodiments, aphotoelectrochemical cell may be illuminated with light (e.g. solarsimulated AM 1.5 radiation) to generate a photocurrent. The overallenergy conversion efficiency of the device may be determined by Equation17:

η(%)=100(E−V _(bias))(i _(t))/(P _(hv) A)  (17)

wherein η is the overall energy conversion efficiency of the device, Eis the Nernstian value for electrolysis of the solution redox species(e.g., conversion of water to hydrogen and oxygen gas), V_(bias) is thevoltage across the cell, i_(t) is the total current flowing in thedevice, P_(hv) is the power of the incident light radiation, and A isirradiated surface area. V_(bias) is generally defined to be negative ifthe cell can simultaneously produce electrical power and stored chemicalenergy, and is generally defined to be positive if an additional powerinput is needed for the cell to perform the desired electrolysisreaction. In some embodiments, the overall energy conversion efficiencymay be less than about 0.1%, less than about 1%, less than about 2%,less than about 5%, less than about 10%, less than about 15%, less thanabout 18%, less than about 20%, less than about 25%, less than about30%, less than about 50%, or the like. In some cases, the overall energyconversion efficiency is about 0.1%, about 0.5%, about 1%, about 5%,about 10%, about 15%, about 18%, about 20%, about 25%, about 30%, about35%, about 40%, about 50%, or the like, or between about 0.1% and about30%, between about 1% and about 30%, between about 10% and about 50%,between about 10% and about 30%, or any range therein. Those of ordinaryskill in the art will be aware of techniques for determining the overallenergy conversion efficiency, for example, see Parkinson et al., Acc.Chem. Res. 1984, 17, 431-437.

The catalytic material (and/or the photoanode comprising the catalyticmaterial) may also be characterized in terms of performance. One way ofdoing this, among many, is to compare the current density of thephotoanode versus the photoactive electrode alone. Typical photoactiveelectrodes are described more fully below and can include titaniumdioxide (e.g., TiO₂), and the like. The photoactive electrode may beable to function, itself, as a photoanode in water electrolysis, and mayhave been used in the past to do so. So, the current density duringcatalytic water electrolysis (where the photoanode catalyticallyproduces oxygen gas from water), using the photoactive electrode, ascompared to essentially identical conditions (with the same counterelectrode or photocathode, same electrolyte, same external circuit, samewater source, etc.), using the photoanode including both photoactiveelectrode and catalytic material, can be compared. In most cases, thecurrent density of the photoanode is greater than the current density ofthe photoactive electrode alone, where each is tested independentlyunder essentially identical conditions. For example, the current densityof the photoanode may exceed the current density of the photoactiveelectrode by a factor of at least about 10, about 100, about 1000, about10⁴, about 10⁵, about 10⁶, about 10⁸, about 10¹⁰, and the like. In aparticular case, the difference in the current density is at least about10⁵. In some embodiments, the current density of the photoanode mayexceed the current density of the photoactive electrode by a factorbetween about 10⁴ and about 10¹⁰, between about 10⁵ and about 10⁹, orbetween about 10⁴ and about 10⁸. The current density may either be thegeometric current density or the total current density, as describedherein.

This characteristic, namely, significantly increased catalytic activityof the photoanode (comprising a photoactive electrode and catalyticmaterial associated with the photoactive electrode) as compared to thephotoactive electrode alone, may be used to monitor formation of acatalytic photoanode. That is, the formation of the catalytic materialon the photoactive electrode may be observed by monitoring the currentdensity over a period of time. The current density, in most cases,increases during application of a voltage to the photoactive electrode.In some instances, the current density may reach a plateau after aperiod of time (e.g., about 2 hours, about 4 hours, about 6 hours, about8 hours, about 10 hours, about 12 hours, about 24 hours, and the like).

Metal ionic species useful as one portion of a catalytic material of theinvention may be any metal ion selected according to the guidelinesdescribed herein. In most embodiments, the metal ionic species haveaccess to oxidation states of at least (n) and (n+x). In some cases, themetal ionic species have access to oxidation states of (n), (n+1) and(n+2). (n) may be any whole number, and includes, but is not limited to,0, 1, 2, 3, 4, 5, 6, 7, 8, and the like. In some cases, (n) is not zero.In particular embodiments, (n) is 1, 2, 3 or 4. (x) may be any wholenumber and includes, but is not limited to 0, 1, 2, 3, 4, and the like.In particular embodiments, (x) is 1, 2, or 3. Non-limiting examples ofmetal ionic species include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr,Nb, Mo, Tc, Rh, Ru, Ag, Cd, Pt, Pd, Ir, Hf, Ta, W, Re, Os, Hg, and thelike. In some cases, the metal ionic species may be a lanthanide oractinide (e.g., Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th,Pa, U, etc.). In a particular embodiment, the metal ionic speciescomprises cobalt ions, which may be provided as a catalytic material inthe form of Co(II), Co(III) or the like. In some embodiments, the metalionic species is not Mn. The metal ionic species may be provided (e.g.,to the solution) as a metal compound, wherein the metal compoundcomprises metal ionic species and counter anions. For example, the metalcompound may be an oxide, a nitrate, a hydroxide, a carbonate, aphosphite, a phosphate, a sulphite, a sulphate, a triflate, and thelike.

An anionic species selected for use as a catalytic material of theinvention may be any anionic species that is able to interact with themetal ionic species as described herein and to meet threshold catalyticrequirements as described. In some cases, the anionic compound may beable to accept and/or donate hydrogen ions, for example, H₂PO₄ ⁻ or HPO₄⁻². Non-limiting examples of anionic species include forms of phosphate(H₃PO₄ or HPO₄ ⁻², H₂PO₄ ⁻² or PO₄ ⁻³), forms of sulphate (H₂SO₄ or HSO₄⁻, SO₄ ⁻²), forms of carbonate (H₂CO₃ or HCO₃ ⁻, CO₃ ⁻²), forms ofarsenate (H₃AsO₄ or HAsO₄ ⁻², H₂AsO₄ ⁻² or AsO₄ ⁻³), forms of phosphite(H₃PO₃ or HPO₃ ⁻², H₂PO₃ ⁻² or PO₃ ⁻³), forms of sulphite (H₂SO₃ or HSO₃⁻, SO₃ ⁻²), forms of silicate, forms of borate (e.g., H₃BO₃, H₂BO₃ ⁻,HBO₃ ⁻², etc.), forms of nitrates, forms of nitrites, and the like.

In some cases, the anionic species may be a form of phosphonate. Aphosphonate is a compound comprising the structure PO(OR¹)(OR²)(R³)wherein R¹, R², and R³ can be the same or different and are H, an alkyl,an alkenyl, an alkynyl, a heteroalkyl, a heteroalkenyl, a heteroalkynyl,an aryl, or a heteroaryl, all optionally substituted, or are optionallyabsent (e.g., such that the compound is an anion, dianion, etc.). In aparticular embodiment, R¹, R², and R³ can be the same or different andare H, alkyl, or aryl, all optionally substituted. A non-limitingexample of a phosphonate is a form of PO(OH)₂R¹ (e.g., PO₂(OH)(R¹)⁻,PO₃(R¹)⁻²), wherein R¹ is as defined above (e.g., alkyl such as methyl,ethyl, propyl, etc.; aryl such as phenol, etc.). In a particularembodiment, the phosphonate may be a form of methyl phosphonate(PO(OH)₂Me), or phenyl phosphonate (PO(OH)₂Ph). Other non-limitingexamples of phosphorus-containing anionic species include forms ofphosphinites (e.g., P(OR¹)R²R³) and phosphonites (e.g., P(OR¹)(OR²)R³)wherein R¹, R², and R³ are as described above. In other cases, theanionic species may comprise one any form of the following compounds:R¹SO₂(OR²)), SO(OR¹)(OR²), CO(OR¹)(OR²), PO(OR¹)(OR²),AsO(OR¹)(OR²)(R³), wherein R¹, R², and R³ are as described above. Withrespect to the anionic species discussed above, those of ordinary skillin the art will be able to determine appropriate substituents for theanionic species. The substituents may be chosen to tune the propertiesof the catalytic material and reactions associated with the catalyticmaterial. For example, the substituent may be selected to alter thesolubility constant of a composition comprising the anionic species andthe metal ionic species.

In some embodiments, the anionic species may be good proton-acceptingspecies. As used herein, a “good proton-accepting species” is a specieswhich acts as a good base at a specified pH level. For example, aspecies may be a good proton-accepting species at a first pH and a poorproton-accepting species at a second pH. Those of ordinary skill in theart can identify a good base in this context. In some cases, a good basemay be a compound in which the pK_(a) of the conjugate acid is greaterthan the pK_(a) of the proton donor in solution. As a specific example,SO₄ ⁻² may be a good proton-accepting species at about pH 2.0 and a poorproton-accepting species at about pH 7.0. A species may act as a goodbase around the pK_(a) value of the conjugate acid. For example, theconjugate acid of HPO₄ ⁻² is H₂PO₄ ⁻, which has a pK_(a) value of about7.2. Therefore, HPO₄ ⁻² may act as a good base around pH 7.2. In somecases, a species may act as a good base in solutions with a pH level atleast about 4 pH units, about 3 pH units, about 2 pH units, or about 1pH unit, above and/or below the pK_(a) value of the conjugate acid.Those of ordinary skill in the art will be able to determine at which pHlevels an anionic species is a good proton-accepting species.

The anionic species may be provided as a compound comprising the anionicspecies and a counter cation. The counter cation may be any cationicspecies, for example, a metal ion (e.g., K⁺, Na⁺, Li⁺, Mg⁺², Ca⁺²,Sr⁺²), NR₄ ⁺ (e.g., NH₄ ⁺), H⁺, and the like. In a specific embodiment,the compound employed may be K₂HPO₄.

The catalytic material may comprise the metal ionic species and anionicspecies in a variety of ratios (amounts relative to each other). In somecases, the catalytic material comprises the metal ionic species and theanionic species in a ratio of less than about 20:1, less than about15:1, less than about 10:1, less than about 7:1, less than about 6:1,less than about 5:1, less than about 4:1, less than about 3:1, less thanabout 2:1, greater than about 1:1, greater than about 1:2, greater thanabout 1:3, greater than about 1:4, greater than about 1:5, greater thanabout 1:10, and the like. In some cases, the catalytic material maycomprise additional components, such as counter cations and/or counteranions from the metallic compound and/or anionic compound provided tothe solution. For example, in some instances, the catalytic material maycomprise the metal ionic species, the anionic species, and a countercation and/or anion in a ratio of about 2:1:1, about 3:1:1, about 3:2:1,about 2:2:1, about 2:1:2, about 1:1:1, and the like. The ratio of thespecies in the catalytic material will depend on the species selected.In some instances, a counter cation may be present in a very smallamount and serve as a dopant to, for example, to improve theconductivity or other properties of the material. In these instances,the ratio may be about X:1:0.1, about X:1:0.005, about X:1:0.001, aboutX:1:0.0005, etc., where X is 1, 1.5, 2, 2.5, 3, and the like. In someinstances, the catalytic material may additionally comprise at least oneof water, oxygen gas, hydrogen gas, oxygen ions (e.g., O⁻²), peroxide,hydrogen ion (e.g., H⁺), and/or the like.

In some embodiments, a catalytic material of the invention may comprisemore than one type of metal ionic species and/or anionic species (e.g.,at least about 2 types, at least about 3 types, at least about 4 types,at least about 5 types, or more, of metal ionic species and/or anionicspecies). For example, more than one type of metal ionic species and/oranionic species may be provided to the solution in which the photoactiveelectrode is immersed. In such instances, the catalytic material maycomprise more than one type of metal ionic species and/or anionicspecies. Without wishing to be bound by theory, the presence of morethan one type of metal ionic species and/or anionic species may allowfor the properties of the photoanode to be tuned, such that theperformance of the photoanode may be altered by using combinations ofspecies in different ratios. In a particular embodiment, a first type ofmetal ionic species (e.g., Co(II)) and second type of metal ionicspecies (e.g., Ni(II)) may be provided in the solution in which thephotoactive electrode is immersed, such that the catalytic materialcomprises the first type of metal ionic species and the second type ofmetal ionic species (e.g., Co(II) and Ni(II)). Where a first and secondtype of metal ionic species are used together, each can be selected fromamong metal ionic species described as suitable for use herein.

Where both first type and a second type of metal ionic and/or anionicspecies are used, both the first and second species need not both becatalytically active, or if both are catalytically active they need notbe active to the same level or degree. The ratio of the first type ofmetal ionic and/or anionic species to the second type of metal ionicand/or anionic species may be varied and may be about 1:1, about 1:2,about 1:3, about 1:4, about 1:5, about 1:6, about 1:7, about 1:8, about1:9, about 1:10, about 1:20, or greater. In some instances, the secondtype of species may be present in a very small amount and serve as adopant to, for example, to improve the conductivity or other propertiesof the material. In these instances, the ratio of the first type ofspecies to the second type of metal ionic species may be about 1:0.1,about 1:0.005, about 1:0.001, about 1:0.0005, etc. In some embodiments,a catalytic material comprising more than one metal ionic species and/oranionic species may be formed by first forming a catalytic materialcomprising a first type of metal ionic species and a first type ofanionic species, followed by exposing the photoanode comprising thecatalytic material to a solution comprising a second type of metal ionicspecies and/or second type of anionic species and applying a voltage tothe photoanode (e.g., via an external power source or by exposing thephotoanode to electromagnetic radiation). This may cause the second typeof metal ionic species and/or second type of anionic species to becomprised in the catalytic material. In other embodiments, the catalyticmaterial may be formed by exposing a photoactive electrode to a solutioncomprising the components (e.g., first and second type of metal ionicspecies, and anionic species) and applying a voltage to the photoactiveelectrode, thereby forming a catalytic material comprising thecomponents.

In some cases, a first type of anionic species and a second type ofanionic species (e.g., a form of borate and a form of phosphate) may beprovided to the solution and/or otherwise used in combination in acatalytic material of the invention. Where both first and secondcatalytically active anionic species are used, they can be selected fromamong anionic species described as suitable for use herein.

In some instances, the first type of anionic species is hydroxide and/oroxide ions, and the second type of anionic species is not hydroxideand/or oxide ions. It should be understood, however, that when at leasttype of anionic species is an oxide or hydroxide, the species might notbe provided to the solution but instead, may be present in the water orsolution the species is provided in and/or may be formed during areaction (e.g., between the first type of anionic species and the metalionic species).

In some embodiments, the catalytic metal ionic species/anionic speciesdo not consist essentially of metal ionic species/O⁻² and/or metal ionicspecies/OH⁻. A material “consists essentially of” a species if it ismade of that species and no other species that significantly alters thecharacteristics of the material, for purposes of the invention, ascompared to the original species in pure form. Accordingly, where acatalytic material does not consist essentially of metal ionicspecies/O⁻² and/or metal ionic species/OH⁻, the catalytic materials hascharacteristics significantly different than a pure metal ionicspecies/O⁻² and/or metal ionic species/OH⁻, or a mixture. In some cases,a composition that does not consist essentially of metal ionicspecies/O⁻² and/or metal ionic species/OH⁻ comprises less than about90%, less than about 80%, less than about 70%, less than about 60%, lessthan about 50%, less than about 40%, less than about 30%, less thanabout 20%, less than about 10%, less than about 5%, less than about 1%,and the like, weight percent of O⁻² and/or OH⁻ ions/molecules. In someinstances, the composition that does not consist essentially of metalionic species/O⁻² and/or metal ionic species/OH⁻ comprises between about1% and about 99%, between about 1% and about 90%, between about 1% andabout 80%, between about 1% and about 70%, between about 1% and about60%, between about 1% and about 50%, between about 1% and about 25%,etc., weight percent O⁻² and/or OH⁻ ions/molecules. The weight percentof O⁻² and/or OH⁻ ions/molecules may be determined using methods knownto those of ordinary skill in the art. For example, the weight percentmay be determined by determining the approximate structure of thematerial comprise in the composition. The weight percentage of the O⁻²and/or OH⁻ ions/molecules may be determined by dividing the weight ofO⁻² and/or OH⁻ ions/molecules over the total weight of the compositionmultiplied by 100%. As another example, in some cases, the weightpercentage may be approximately determined based upon the ratio of metalionic species to anionic species in a composition and knowledgeregarding the general coordination chemistry of the metal ionic species.

In a specific embodiment, the composition (e.g., catalytic material)associated with the photoactive electrode may comprise cobalt ions andanionic species comprising phosphorus (e.g., HPO₄ ⁻²). In some cases,the composition may additionally comprise cationic species (e.g., K⁺).In some cases, the photoactive electrode with which the composition isassociated does not consist essentially of platinum. An anionic speciescomprising phosphorus may be any molecule that comprises phosphorus andis associated with a negative charge. Non-limiting examples of anionicspecies comprising phosphorus include H₃PO₄, H₂PO₄ ⁻, HPO₄ ⁻², PO₄ ⁻³,H₃PO₃, H₂PO₃ ⁻, HPO₃ ⁻², PO₃ ⁻³, R¹PO(OH)₂, R¹PO₂(OH)⁻, R¹PO₃ ⁻², or thelike, wherein R¹ is H, an alkyl, an alkenyl, an alkynyl, a heteroalkyl,a heteroalkenyl, a heteroalkynyl, an aryl, or a heteroaryl, alloptionally substituted.

In some embodiments, a catalytic material of the invention, especiallywhen associated with the photoactive electrode, may be substantiallynon-crystalline. Without wishing to be bound by theory, a substantiallynon-crystalline material may aid in the transport of protons and/orelectrons, which may improve the function of the photoanode in certainelectrochemical devices. For example, improved transport of protons(e.g., increase proton flux) during electrolysis may improve the overallefficiency of an electrochemical device comprising a photoanode asdescribed herein. A photoanode comprising a substantiallynon-crystalline catalytic material may allow for a conductivity ofprotons of at least about 10⁻¹ S cm⁻¹, at least about 20⁻¹ S cm⁻¹, atleast about 30⁻¹ S cm⁻¹, at least about 40⁻¹ S cm⁻¹, at least about 50⁻¹S cm⁻¹, at least about 60⁻¹ S cm⁻¹, at least about 80⁻¹ S cm⁻¹, at leastabout 100⁻¹ S cm⁻¹, and the like. In other embodiments, the catalyticmaterial may be amorphous, substantially crystalline, or crystalline.Where substantially non-crystalline material is used, this would bereadily understood by those of ordinary skill in the art and easilydetermined using various spectroscopic techniques.

The above and other characteristics of the metal ionic species andanionic species can serve as selective screening tests foridentification of particular metal ionic and anionic species useful forparticular applications. Those of ordinary skill in the art can, throughsimple bench-top testing, reference to scientific literature, simplediffractive instrumentation, simple electrochemical testing, and thelike, select metal ionic and anionic species based upon the presentdisclosure, without undue experimentation.

In some cases, the catalytic material associated with the photoactiveelectrode may be porous, substantially porous, non-porous, and/orsubstantially non-porous. The pores may comprise a range of sizes and/orbe substantially uniform in size. In some cases, the pores may or maynot be visible using imaging techniques (e.g., scanning electronmicroscope). The pores may be open and/or closed pores. In some cases,the pores may provide pathways between the bulk electrolyte surface andthe surface of the photoactive electrode.

In some instances, the catalytic material may be hydrated. That is, thecatalytic material may comprise water and/or other liquid and/or gascomponents. Upon removal of the photoactive electrode comprising thecatalytic material from solution, the catalytic material may bedehydrated (e.g., the water and/or other liquid and/or gas componentsmay be removed from the catalytic material). In some cases, thecatalytic material may be dehydrated by removing the material fromsolution and leaving the material to sit under ambient conditions (e.g.,room temperature, air, etc.) for at least about 1 hour, at least about 2hours, at least about 4 hours, at least about 8 hours, at least about 12hours, at least about 24 hours, at least about 2 days, at least about 1week, or more. In some cases, the catalytic material may be dehydratedunder non-ambient conditions. For example, the catalytic material bedehydrated at elevated temperature and/or under vacuum. In someinstances, the catalytic material may change composition and/ormorphology upon dehydration. For example, in instances where thecatalytic material forms a film, the film may comprise cracks upondehydration.

Without wishing to be bound by theory, in some cases, the catalyticmaterial may reach a maximum performance (e.g., rate of O₂ production,overpotential at a specific current density, onset potential, Faradaicefficiency, etc.) based upon the thickness of the catalytic material.Where a porous photoactive electrode is used, the thickness of thedeposited catalytic material and the pore size of photoactive electrodemay advantageously be selected in combination so that pores are notsubstantially filled with the catalytic material. For example, thesurface of the pores may comprise a layer of the catalytic material thatis thinner than the average radius of the pores, thereby allowing forsufficient porosity to remain, even after catalytic material isdeposited, so that the high surface area provided by the porousphotoactive electrode is substantially maintained. In some cases, theaverage thickness of the catalytic material may be less than about 90%,less than about 80%, less than about 70%, less than about 60%, less thanabout 50%, less than about 40%, less than about 30%, less than about20%, less than about 10%, or less, the average radius of the pores ofthe photoactive electrode. In some cases, the average thickness of thecatalytic material may be between about 40% and about 60%, between about30% and about 70%, between about 20% and about 80%, etc., the averageradius of the pores of the photoactive electrode. In other embodiments,the performance of the catalytic material might not reach a maximumperformance based upon the thickness of the catalytic material. In otherembodiments, the performance of the catalytic material might not reach amaximum performance based upon the thickness of the catalytic material.In some cases, the performance (e.g., overpotential at a certain currentdensity may decrease) of the catalytic material may increase withincreasing thickness of the catalytic material. Without wishing to bebound by theory, this may indicate greater than just the outside layerof the catalytic material is catalytically active.

The physical structure of the catalytic material may vary. For example,the catalytic material may be a film and/or particles associated with atleast a portion of the photoactive electrode (e.g., surface and/orpores) that is immersed in the solution. In some embodiments, thecatalytic material might not form a film associated with the photoactiveelectrode. Alternatively or in addition, the catalytic material may bedeposited on a photoactive electrode as patches, islands, or some otherpattern (e.g., lines, spots, rectangles), or may take the form ofdendrimers, nanospheres, nanorods, or the like. A pattern in some casescan form spontaneously upon deposition of catalytic material onto thephotoactive electrode and/or can be patterned onto a photoactiveelectrode by a variety of techniques known to those of ordinary skill inthe art (lithographically, via microcontact printing, etc.) and asdiscussed herein. Further, a photoactive electrode may be patterneditself such that certain areas facilitate association of the catalyticmaterial while other areas do not, or do so to a lesser degree, therebycreating a patterned arrangement of catalytic material on thephotoactive electrode as the photoanode is formed. Where a catalyticmaterial is patterned onto a photoanode, the pattern might define areasof catalytic material and areas completely free of catalytic material,or areas with a particular amount of catalytic material and other areaswith a different amount of catalytic material deposition. The catalyticmaterial may have an appearance of being smooth and/or bumpy. In somecases, the catalytic material may comprise cracks, as can be the casewhen the material dehydrated.

In some cases, the thickness of catalytic material may be ofsubstantially the same throughout the material. In other cases, thethickness of the catalytic material may vary throughout the material(e.g., a film does not necessarily have uniform thickness). Thethickness of the catalytic material may be determined by determining thethickness of the material at a plurality of areas (e.g., at least 2, atleast 4, at least 6, at least 10, at least 20, at least 40, at least 50,at least 100, or more areas) and calculating the average thickness.Where thickness of a catalytic material is determined via probing at aplurality of areas, the areas should be selected so as not tospecifically represent areas of more or less catalytic material presencebased upon a pattern. Those of ordinary skill in the art will easily beable to establish a thickness-determining protocol that accounts for anynon-uniformity or patterning of catalytic material on the surface. Forexample, the technique might include a sufficiently large number of areadeterminations, randomly selected, to provide overall average thickness.The average thickness of the catalytic material may be at least about 10nm, at least about 100 nm, at least about 300 nm, at least about 500 nm,at least about 700 nm, at least about 1 um (micrometer), at least about2 um, at least about 5 um, at least about 1 mm, at least about 1 cm, andthe like. In some cases, the average thickness of the catalytic materialmay be less than about 1 mm, less than about 500 um, less than about 100um, less than about 10 um, less than about 1 um, less than about 100 nm,less than about 10 nm, less than about 1 nm, less than about 0.1 nm, orthe like. In some instances, the average thickness of the catalyticmaterial may be between about 1 mm and about 0.1 nm, between about 500um and about 1 nm, between about 100 um and about 1 nm, between about100 um and about 0.1 nm, between about 0.2 um and about 2 um, betweenabout 200 um and about 0.1 um, or the like. In particular embodiments,the catalytic material may have an average thickness of less than about0.2 um. In another embodiment, the catalytic material may have anaverage thickness between about 0.2 um and about 2 um. The averagethickness of the catalytic material may be varied by altering the amountand length of time a voltage is applied to the photoactive electrode,the concentration of the metal ionic species and the anionic species insolution, the surface area of the photoactive electrode, the surfacearea density of the photoactive electrode, and the like.

In some cases, the average thickness of the catalytic material may bedetermined according to the following method. A photoanode comprising aphotoactive electrode and a catalytic material may be removed fromsolution (e.g., the solution the photoanode was formed in and/or theelectrolyte). The photoanode may be left to dry for about 1 hour, about2 hours, about 4 hours, about 6 hours, about 8 hours, about 12 hours,about 24 hours, or more. In some cases, the photoanode may be driedunder ambient conditions (e.g., in air at room temperature). In someembodiments, during drying, the catalytic material may crack. Thethickness of the catalytic material may be determined using techniquesknown to those of ordinary skill in the art (e.g., scanning electronmicroscope (SEM)) to determine the depth of the cracks (e.g., thethickness of the dehydrated catalytic material).

In other embodiments, the thickness of the catalytic material may bedetermined without dehydration (e.g., in situ) using techniques known tothose of ordinary skill in the art, for example, SEM. In suchembodiments, a mark (e.g., scratch, hole) may be made in the catalyticmaterial to expose at least a portion of the underlying substrate (e.g.,the photoactive electrode). The thickness of the catalytic material maybe determined by measuring the depth of the mark.

In some embodiments, a film of the catalytic material may be formed bythe coalescing of a plurality of particles formed on the photoactiveelectrode. In some cases, the material may be observed to have thephysical appearance of a base layer of material comprising a pluralityof groups of protruding particles. The thickness of the film may bedetermined by the thickness of the base layer, although it should beunderstood that the thickness would be substantially greater if measuredby determining the thickness of the areas comprising protrudingparticles.

Without wishing to be bound by theory, the formation of groups ofprotruding particles on the surface of the film may aid in increasingthe surface area and thus increase the production of oxygen gas. Thatis, the surface area of the catalytic material comprising a plurality ofgroups of protruding particles may be substantially greater than thesurface area of a catalytic material which does not comprise a pluralityof groups of protruding particles.

In some embodiments, the catalytic material may be described as afunction of mass of catalytic material per unit area of the photoactiveelectrode. In some cases, the mass of catalytic material per area of thephotoactive electrode may be about 0.01 mg/cm², about 0.05 mg/cm², about0.1 mg/cm², about 0.5 mg/cm², about 1.0 mg/cm², about 1.5 mg/cm², about2.5 mg/cm², about 3.0 mg/cm², about 4.0 mg/cm², about 5.0 mg/cm², or thelike. In some cases, the mass of catalytic material per unit area of thephotoactive electrode may be between about 0.1 mg/cm² and about 5.0mg/cm², between about 0.5 mg/cm² and about 3.0 mg/cm², between about 1.0mg/cm² and about 2.0 mg/cm², and the like. Where the amount of catalyticmaterial associated with a photoactive electrode is defined orinvestigated in terms of mass per unit area, and the material is presentnon-uniformly relative to the photoactive electrode surface (whetherthrough patterning or natural variations in amount over the surface),the mass per unit area may be averaged across the entire surface areawithin which catalytic material is found (e.g., the geometric surfacearea). In some cases, the mass of the catalytic material per unit areamay be a function of the thickness of the catalytic material.

The formation of the catalytic material may proceed until the voltageapplied to the photoactive electrode is turned off (e.g., the powersource or the light source is turn off/removed), until there is alimiting quantity of materials (e.g., metal ionic species and/or anionicspecies) and/or the catalytic material has reached a critical thicknessbeyond which additional film formation does not occur or is very slow.Voltage may be applied to the photoactive electrode for minimums ofabout 1 minute, about 5 minutes, about 10 minutes, about 20 minutes,about 30 minutes, about 60 minutes, about 2 hours, about 4 hours, about8 hours, about 12 hours, about 24 hours, and the like. In some cases, apotential may be applied to the photoactive electrode between 24 hoursand about 30 seconds, between about 12 hours and about 1 minute, betweenabout 8 hours and about 5 minutes, between about 4 hours and about 10minutes, and the like. The voltages provided herein, in some cases, aresupplied with reference to a ‘normal hydrogen electrode’ (NHE). Those ofordinary skill in the art will be able to determine the correspondingvoltages with respect to an alternative reference electrode by knowingthe voltage difference between the specified reference electrode and NHEor by referring to an appropriate textbook or reference. The formationof the catalytic material may proceed until about 0.1%, about 1%, about5%, about 10%, about 20%, about 30%, about 40%, about 50%, about 60%,about 70%, about 80%, about 90%, about 99%, about 100% of the metalionic species and/or anionic species initially added to the solutionhave associated with the photoactive electrode to form the catalyticmaterial.

The voltage applied to the photoactive electrode (e.g., via an externalpower source or by exposing the photoactive electrode to electromagneticradiation) may be held steady, may be linearly increased or decreased,and/or may be linearly increased and decreased (e.g., cyclic). In somecases, the voltage applied to the photoactive electrode may besubstantially similar throughout the application of the voltage. Thatis, the voltage applied to the photoactive electrode might not be variedsignificantly during the time that the voltage is applied to thephotoactive electrode. In some instances, the voltage applied to thecurrent collect by an external power source may be at least about 0.1 V,at least about 0.2 V, at least about 0.4 V, at least about 0.5 V, atleast about 0.7 V, at least about 0.8 V, at least about 0.9 V, at leastabout 1.0 V, at least about 1.2 V, at least about 1.4 V, at least about1.6 V, at least about 1.8 V, at least about 2.0 V, at least about 3 V,at least about 4 V, at least about 5 V, at least about 10 V, and thelike. In some cases, the voltage applied is between about 1.0 V andabout 1.5 V, about 1.1 V and about 1.4 V, or is about 1.1 V. In someinstances, the voltage applied to the photoactive electrode may be alinear range of voltages, and/or cyclic range of voltages. Applicationof a linear voltage refers to instances where the voltage applied to thephotoanode (and/or photoactive electrode) is swept linearly in timebetween a first voltage and a second voltage. Application of a cyclicvoltage refers to application of linear voltage, followed by a secondapplication of linear voltage wherein the sweep direction has beenreversed. For example, application of a cyclic voltage is commonly usedin cyclic voltammetry studies. In some cases, the first voltage and thesecond voltage may differ by about 0.1 V, about 0.2 V, about 0.3 V,about 0.5 V, about 0.8 V, about 1.0 V, about 1.5 V, about 2.0 V, or thelike. In some cases, the voltage may be swept between the first voltageand the second voltage at a rate of about 0.1 mV/sec, about 0.2 mV/sec,about 0.3 mV/sec, about 0.4 mV/sec, about 0.5 mV/sec, about 1.0 mV/sec,about 10 mV/sec, about 100 mV/sec, about 1 V/sec, or the like. Thepotential applied may or might not be such that oxygen gas is beingformed during the formation of the photoanode. In some cases, themorphology of the catalytic material may differ depending on thepotential applied to the photoactive electrode during formation of thephotoanode.

In some embodiments, wherein the catalytic material is a regenerativematerial, between application of a voltage (e.g., during periods whenthe photoanode is not in use), at least about 1%, at least about 2%, atleast about 5%, at least about 10%, at least about 20%, or more, byweight of the catalytic material may dissociate from the photoactiveelectrode over a period of about 10 minutes, about 30 minutes, about 1hour, about 2 hours, about 6 hours, about 12 hours, about 24 hours, ormore. Upon reapplication of the voltage, at least about 50%, at leastabout 60%, at least about 70%, at least about 80%, at least about 90%,at least about 95%, at least about 99%, or more, by weight of thedissociate material may re-associate with the photoanode. In some cases,substantially all of the metal ionic species may re-associate with thephotoanode and only a portion of the anionic species may re-associatewith the photoanode (e.g., in instances where the electrolyte comprisesanionic species and there may be an exchange of the anionic specieswhich dissociate and those which re-associate).

In another embodiment, a photoanode of system comprising a catalyticmaterial may be prepared as follows. A catalytic material may beassociated with a photoactive electrode as described above in any mannerdescribed herein. For example, at relatively low potentials at whichoxygen gas is not evolved, and/or at a higher potentials at which oxygengas is evolved and a higher rate of deposition of material on thephotoanode occurs, and/or at any other rate or under any conditionssuitable for production of a catalytic material associated with thephotoactive electrode. The catalytic material can be removed from thephotoactive electrode (and, optionally, the process can be cyclicallyrepeated with additional catalytic material associated with thephotoanode, removed, etc.) and the catalytic material can be optionallydried, stored, and/or mixed with an additive (e.g., a binder) or thelike. The catalytic material may be packaged for distribution and usedas a catalytic material. In some cases, the catalytic material can laterbe applied to a photoactive electrode, can simply be added to a solutionof water and associated with a different photoactive electrode asdescribed above, e.g., in an end-use setting, or used otherwise as wouldbe recognized by those of ordinary skill in the art. Those of ordinaryskill in the art can readily select binders that would be useful foraddition to such catalytic material, for example, polytetrafluoroethylene (Teflon™), Nafion™, or the like. For eventual use inan electrolyzer, photoelectrochemical cell, or other electrolysissystem, non-conductive binders may be most suitable. Conductive bindersmay be used where they are stable to photoelectrochemical conditions.

In some embodiments, after application of the voltage and formation of aphotoanode comprising a photoactive electrode, metal ionic species, andanionic species, the photoanode may be removed from the solution andstored. The photoanode may be stored for any period of time or usedimmediately in one of the applications discussed herein. In some cases,the catalytic material associated with the photoactive electrode maydehydrate during storage. The photoanode may be stored for at leastabout 1 day, at least about 2 days, at least about 5 days, at leastabout 10 days, at least about 1 month, at least about 3 months, at leastabout 6 months or at least about 1 year, with no more than 10% loss inphotoanode performance per month of storage, or no more than 5%, or even2%, loss in performance per month of storage. Photoanodes as describedherein may be stored under varying conditions. In some instances, thephotoanode may be stored in ambient conditions and/or under anatmosphere of air. In other instances, the photoanode may be storedunder vacuum. In yet another instance, the photoanode may be stored insolution. In this case, the catalytic material may disassociate from thephotoactive electrode over a period of time (e.g., 1 day, 1 week, 1month, and the like) to form metal ionic species and anionic species insolution. Application of a voltage and/or a photovoltage to thephotoactive electrode, in most cases, may cause the metal ionic speciesand anionic species to re-associate with the photoactive electrode toreform the catalytic material.

In some embodiments, a photoanode comprising a photoactive electrode anda catalytic material may be used for an extended period of time ascompared to the photoactive electrode alone, under essentially identicalconditions. Without wishing to be bound by theory, the dynamicequilibrium of the catalytic material may cause the photoanode to berobust and provides a self-repair mechanism. In some cases, a photoanodemay be used to catalytically produce oxygen gas from water for at leastabout 1 month, at least about 2 months, at least about 3 months, atleast about 6 months, at least about 1 year, at least about 18 months,at least about 2 years, at least about 3 years, at least about 5 years,at least about 10 years, or greater, with less than 50%, less than 40%,less than 30%, less than 20%, less than 10%, less than 5%, less than 3%,less than 2%, less than 1%, or less, change in a selected performancemeasure (e.g., overpotential, rate of production of oxygen, etc.).

In some cases, the composition of the catalytic material associated withthe photoactive electrode after storage may be substantially similar tothe catalytic material immediately after formation. In other cases, thecomposition of the catalytic material associated with the photoactiveelectrode after storage may be substantially different than thecatalytic material immediately after formation. In some instances, themetal ionic species in the catalytic material may be oxidized ascompared to the metal ionic species in solution. For example, the metalionic species immediately after deposition may have an oxidation stateof (n+x), and after storage, at least a portion of the metal ionicspecies may have an oxidation state of (n). The ratio of metal ionicspecies to anionic species in the catalytic material after storage mayor might not be substantially similar to the ratio present immediatelyafter formation.

The solution in which the photoactive electrode is immersed may beformed from any suitable material. In most cases, the solution may be aliquid and may comprise water. In some embodiments the solution mayconsist of or consist essentially of water, i.e. be essentially purewater or an aqueous solution that behaves essentially identical to purewater, in each case, with the minimum electrical conductivity necessaryfor an electrochemical device to function. In some embodiments, thesolution may be selected such that the metal ionic species and theanionic species are substantially soluble. In some cases, when thephotoanode is to be used in a device immediately after formation, thesolution may be selected such that it comprises water (or other fuel) tobe oxidized by a device and/or method as described herein. For example,in instances where oxygen gas is to be catalytically produced fromwater, the solution may comprise water (e.g., provided from a watersource). In some cases, the solution may be contained within a containerwhich is substantially transparent to visible light (e.g., such that thephotoactive electrode may be exposed to electromagnetic radiationthrough the container).

The metal ionic species and the anionic species may be provided to thesolution by substantially dissolving compounds comprising the metalionic species and the anionic species. In some instances, this maycomprise substantially dissolving a metal compound comprising the metalionic species and anionic compound comprising the anionic species. Inother instances, a single compound may be dissolved that comprises boththe metal ionic species and the anionic species. The metal compoundand/or the anionic compound may be of any composition, such as a solid,a liquid, a gas, a gel, a crystalline material, and the like. Thedissolution of the metal compound and anionic compound may befacilitated by agitation of the solution (e.g., stirring) and/or heatingof the solution. In some cases, the solution may be sonicated. The metalspecies and/or anionic species may be provided in an amount such thatthe concentration of the metal ionic species and/or anionic species isat least about 0.1 mM, at least about 0.5 mM, at least about 1 mM, atleast about 10 mM, at least about 0.1 M, at least about 0.5 M, at leastabout 1 M, at least about 2 M, at least about 5M, and the like. In somecases, the concentration of the anionic species may be greater than theconcentration of the metal ionic species, so as to facilitate theformation of the catalytic material, as described herein. Asnon-limiting examples, the concentration of the anionic species may beabout 2 times greater, about 5 times greater, about 10 times greater,about 25 times greater, about 50 times greater, about 100 times greater,about 500 times greater, about 1000 times greater, and the like, of theconcentration of the metal ionic species. In some instances, theconcentration of the metal ionic species will be greater than theconcentration of the anionic species.

In some cases, the pH of the solution may be about neutral. That is, thepH of the solution may be between about 6.0 and about 8.0, between about6.5 and about 7.5, and/or the pH is about 7.0. In other cases, the pH ofthe solution is about neutral or acidic. In these cases, the pH may bebetween about 0 and about 8, between about 1 and about 8, between about2 and about 8, between about 3 and about 8, between about 4 and about 8,between about 5 and about 8, between about 0 and about 7.5, betweenabout 1 and about 7.5, between about 2 and about 7.5, between about 3and about 7.5, between about 4 and about 7.5, or between about 5 andabout 7.5. In yet other cases, the pH may be between about 6 and about10, between about 6 and about 11, between about 7 and about 14, betweenabout 2 and about 12, and the like. In some embodiments, the pH of thesolution may be about neutral and/or basic, for example, between about 7and about 14, between about 8 and about 14, between about 8 and about13, between about 10 and about 14, greater than 14, or the like. The pHof the solution may be selected such that the anionic species and themetal ionic species are in the desired state. For example, some anionicspecies may be affected by a change in pH level, for example, phosphate.If the solution is basic (greater than about pH 12), the majority of thephosphate is the form PO₄ ⁻³. If the solution is approximately neutral,the phosphate is in approximately equal amounts of the form HPO₄ ⁻² andthe form H₂PO₄ ⁻¹. If the solution is slightly acidic (less than aboutpH 6), the phosphate is mostly in the form H₂PO₄ ⁻. The pH level mayalso affect the solubility constant for the anionic species and themetal ionic species.

In one embodiment, a photoanode as described herein may comprise aphotoactive electrode and a composition comprising metal ionic speciesand anionic species in electrical communication with the photoactiveelectrode. The composition, in some cases, may be formed byself-assembly of the metal ionic species and anionic species on thephotoactive electrode and be sufficiently non-crystalline such that thecomposition allows for the conduction of protons. In some embodiments, aphotoanode may allow for a conductivity of protons of at least 10⁻¹ Scm⁻¹, at least about 20⁻¹ S cm⁻¹, at least about 30⁻¹ S cm⁻¹, at leastabout 40⁻¹ S cm⁻¹, at least about 50⁻¹ S⁻¹ cm⁻¹, at least about 60 Scm⁻¹, at least about 80 S cm⁻¹, at least about 100 S cm⁻¹, and the like.

In some embodiments, a photoanode as described herein may be capable ofproducing oxygen gas from water at a low overpotential. Voltage inaddition to a thermodynamically determined reduction or oxidationpotential that is required to attain a given catalytic activity isherein referred to as “overpotential,” and may limit the efficiency ofthe electrochemical device (e.g., photoelectrochemical device).Overpotential is therefore given its ordinary meaning in the art, thatis, it is the potential that must be applied to a component of a systemsuch as a photoanode to bring about a electrochemical reaction (e.g.,formation of oxygen gas from water) minus the thermodynamic potentialrequired for the reaction. Those of ordinary skill in the art understandthat the total potential that must be applied to a particular system inorder to drive a reaction is typically the total of the potentials thatmust be applied to the various components of the system. For example,the potential for an entire system is typically higher than thepotential as measured at, e.g., a photoanode at which oxygen gas isproduced from the electrolysis of water. Those of ordinary skill in theart will recognize that where overpotential for oxygen production fromwater electrolysis is discussed herein, this applies to the voltagerequired for the conversion of water to oxygen itself, and does notinclude voltage drop at the counter electrode.

The thermodynamic potential for the production of oxygen gas from watervaries depending on the conditions of the reaction (e.g., pH,temperature, pressure, etc.). Those of ordinary skill in the art will beable to determine the required thermodynamic potential for theproduction of oxygen gas from water depending on the experimentalconditions. For example, the pH dependence of water oxidation may bedetermined from a simplified form of the Nernst equation to giveEquation 18:

E _(pH) =E ^(o)−0.059V×(pH)  (18)

where E_(pH) is the potential at a given pH, E^(o) is the potentialunder standard conditions (e.g., 1 atm, about 25° C.) and pH is the pHof the solution. For example, at pH 0, E=1.229 V, at pH 7, E=0.816 V,and at pH 14, E=0.403 V.

The thermodynamic potential for the production of oxygen gas from waterat a specific temperature (E_(T)) may be determined using Equation 19:

E _(T)=[1.5184−(1.5421×10⁻³)(T)]+[(9.523×10⁻⁵)(T)(ln(T))]+[(9.84×10⁻⁸)T²]  (19)

where T is given in Kelvin. For example, at 25° C., E_(T)=1.229 V, andat 80° C., E_(T)=1.18 V.

The thermodynamic potential for the production of oxygen gas from waterat a given pressure (E_(p)) may be determined using Equation 20:

$\begin{matrix}{E_{P} = {E_{T} + {( \frac{RT}{2F} )\ln \{ {\lbrack ( {P - P_{w}} )^{1.5} \rbrack \div ( \frac{P_{w}}{P_{wo}} )} \}}}} & (20)\end{matrix}$

where T is in Kelvin, F is Faraday's constant, R is the universal gasconstant, P is the operating pressure of the electrolyzer, P_(w) is thepartial pressure of water vapor over the chosen electrolyte, and P_(wo)is the partial pressure of water vapor over pure water. By thisequation, at a 25° C., the E_(P) increases by 43 mV for a tenfoldincrease in pressure.

In some instances, a photoanode as described herein may be capable ofcatalytically producing oxygen gas from water (e.g., gaseous and/orliquid water) with an overpotential of less than about 1 volt, less thanabout 0.75 volts, less than about 0.6 volts, less than about 0.5 volts,less than about 0.4 volts, less than about 0.35 volts, less than about0.325 volts, less than about 0.3 volts, less than about 0.25 volts, lessthan about 0.2 volts, less than about 0.1 volts, or the like. In someembodiments, the overpotential is between about 0.1 volts and about 0.4volts, between about 0.2 volts and about 0.4 volts, between about 0.25volts and about 0.4 volts, between about 0.3 volts and about 0.4 volts,between about 0.25 volts and about 0.35 volts, or the like. In anotherembodiment, the overpotential is about 0.325 volts. In some cases, theoverpotential of a photoanode is determined under standardizedconditions of an electrolyte with a neutral pH (e.g., about pH 7.0),ambient temperature (e.g., about 25° C.), ambient pressure (e.g., about1 atm), a photoactive electrode that is non-porous and planar, and at ageometric current density (as described herein) of about 1 mA/cm². It isto be understood that systems of the invention can be used underconditions other than those described immediately above and in factthose of ordinary skill in the art will recognize that a very widevariety of conditions can exist in use of the invention. But theconditions noted above are provided only for the purpose of specifyinghow features such as overpotential, amount of oxygen and/or hydrogenproduced, and other performance characteristics defined herein aremeasured for purposes of clarity of the present invention. In a specificembodiment, a catalytic material may produce oxygen gas from water at anoverpotential of less than 0.4 volt at an electrode current density ofat least 1 mA/cm². As described herein, the water which is oxidized maycontain at least one impurity (e.g., NaCl), or be provided from animpure water source.

In some embodiment, a photoanode may be capable of catalyticallyproducing oxygen gas from water (e.g., gaseous and/or liquid water) witha Faradaic efficiency of about 100%, greater than about 99.8%, greaterthan about 99.5%, greater than about 99%, greater than about 98%,greater than about 97%, greater than about 96%, greater than about 95%,greater than about 90%, greater than about 85%, greater than about 80%,greater than about 70%, greater than about 60%, greater than about 50%,etc. The term, “Faradaic efficiency,” as used herein, is given itsordinary meaning in the art and refers to the efficacy with which charge(e.g., electrons) are transferred in a system facilitating anelectrochemical reaction. Loss in Faradaic efficiency of a system may becaused, for example, by the misdirection of electrons which mayparticipate in unproductive reactions, product recombination, shortcircuit the system, and other diversions of electrons and may result inthe production of heat and/or chemical byproducts.

Faradaic efficiency may determined, in some cases, through bulkelectrolysis where a known quantity of reagent is stoichiometricallyconverted to product as measured by the current passed and this quantitymay be compared to the observed quantity of product measured throughanother analytical method. For example, a device or photoanode may beused to catalytically produce oxygen gas from water. The total amount ofoxygen produced may be measured using techniques know to those ofordinary skill in the art (e.g., using an oxygen sensor, a zirconiasensor, electrochemical methods, etc.). The total amount of oxygen thatis expected to be produced may be determined using simple calculationsfrom the amount of charge passed. The Faradaic efficiency may bedetermined by measuring the percentage of oxygen gas produced andcomparing that value with the expected amount of oxygen gas producedbased on the charge passed during photo-assisted electrolysis. In somecases, the Faradaic efficiency of a photoanode changes by less thanabout 0.1%, less than about 0.2%, less than about 0.3%, less than about0.4%, less than about 0.5%, less than about 1.0%, less than about 2.0%,less than about 3.0%, less than about 4.0%, less than about 5.0%, etc.,over a period of operation of the photoanode of about 1 day, about 2days, about 3 days, about 5 days, about 15 days, about 1 month, about 2months, about 3 months, about 6 months, about 12 months, about 18months, about 2 years, etc.

As will be known to those of ordinary skill in the art, an example of aside reaction that may occur during the catalytic formation of oxygengas from water is the production of hydrogen peroxide. The production ofhydrogen peroxide may decrease the Faradaic efficiency of a photoanode.In some cases, a photoanode, in use, may produce oxygen that is in theform of hydrogen peroxide of less than about 0.01%, less than about0.05%, less than about 0.1%, less than about 0.2%, less than about 0.3%,less than about 0.4%, less than about 0.5%, less than about 0.6%, lessthan about 0.7%, less than about 0.8%, less than about 0.9%, less thanabout 1%, less than about 1.5%, less than about 2%, less than about 3%,less than about 4%, less than about 5%, less than about 10%, etc. Thatis, less than this percentage of the molecules of oxygen produced is inthe form of hydrogen peroxide. Those of ordinary skill in the art willbe aware of methods for determining the production of hydrogen peroxideat a photoanode and/or methods to determine the percentage of hydrogenperoxide produced. For example, hydrogen peroxide may be determinedusing a rotating ring-disc electrode. Any products generated at the diskelectrode are swept past the ring electrode. The potential of the ringelectrode may be poised to detect hydrogen peroxide that may have beengenerated at the ring.

In some cases, the performance of a photoanode may also be expressed, insome embodiments, as a turnover frequency. The turnover frequency refersto the number of oxygen molecules produced per second per catalyticsite. In some cases, a catalytic site may be a metal ionic species(e.g., a cobalt ion). The turnover frequency of a photoanode (e.g.,comprising a photoactive electrode and a catalytic material) may be lessthan about 0.01, less than about 0.005, less than about 0.001, less thanabout 0.0007, less than about 0.0005, less than about 0.00001, less thanabout 0.000005, or less, moles of oxygen gas per second per catalyticsite. In some cases, the turnover frequency may be determined understandardized conditions (e.g., ambient temperature and pressure, 1mA/cm², planar photoactive electrode, etc.). Those of ordinary skill inthe art will be aware of methods to determine the turnover frequency.

In one set of embodiments, the invention provides a photoanode and/orcatalytic system which can facilitate photo-assisted electrolysis (orother electrochemical reactions) wherein a significant portion, oressentially all of the electrons provided to or withdrawn from asolution or material undergoing electrolysis are provided throughreaction of catalytic material. For example, where essentially all theelectrons provided to or withdrawn from a system undergoing electrolysisare involved in a catalytic reaction, essentially each electron added orwithdrawn participates in a reaction involving change of a chemicalstate of at least one element of a catalytic material. In otherembodiments, the invention provides a system where at least about 98%,at least about 95%, at least about 90%, at least about 80%, at leastabout 70%, at least about 60%, at least about 50%, at least about 40%,or at least about 30% of all electrons added to or withdrawn from asystem undergoing electrolysis (e.g., water being split) are involved ina catalytic reaction. Where less than essentially all electrons added orwithdrawn are involved in a catalytic reaction some electrons can simplybe provided to and withdrawn from the electrolysis solution or material(e.g., water) directly to and from a photoactive electrode and/orphotoanode which does participate in a catalytic reaction.

In some embodiments, systems and/or devices may be provided thatcomprise at least one photoanode as described herein and/or preparedusing the methods described herein may be provided. In particular, adevice may be a photoelectrochemical device. Non-limiting examples ofphotoelectrochemical devices includes photoelectrochemical cells,bi-photoelectrochemical cells, hybrid photoelectrochemical cells, andthe like. A photoelectrochemical device in some cases, may function asan oxygen gas and/or hydrogen gas generator by photoelectrochemicallydecomposing water (e.g., liquid and/or gaseous water) to produce oxygenand/or hydrogen gases. Fuel (e.g., water) may be provided to a device ina solid, liquid, gel, and/or gaseous state. In some cases, as describedherein, the oxygen gas and/or hydrogen gas produced may be converted towater using a secondary device, for example, an energy conversion devicesuch as a fuel cell. An energy conversion device, in some embodiments,may be used to provide at least a portion of the energy required tooperate an automobile, a house, a village, a cooling device (e.g., arefrigerator), etc. In some cases, more than one device may be employedto provide the energy.

In some embodiments, a device may be used to produce O₂ and/or H₂. TheO₂ and/or H₂ may be converted back into electricity and water (e.g.,through use of a fuel cell). In some cases, however, the O₂ and/or H₂may be used for other purposes. For example, the O₂ and/or H₂ may beburned to provide a source of heat. In some cases, O₂ may be used incombustion processes (e.g., burning of the hydrocarbon fuels such asoil, coal, petrol, natural gas) which may be used to heat homes, powercars, as rocket fuel, etc. In some instances, O₂ may be used in achemical plant for the production and/or purification of a chemical(e.g., production of ethylene oxide, production of polymers,purification of molten ore). In some cases, the H₂ may be used to powera device (e.g., in a hydrogen fuel cell), wherein the O₂ may be releasedinto the atmosphere and/or used for another purpose. In other cases, H₂may be used for the production of a chemical or in a chemical plant(e.g., for hydrocracking, hydrodealkylation, hydrodesulfurization,hydrogenation (e.g., of fats, oils, etc.), etc.; for the production ofmethanol, acids (e.g., hydrochloric acid), ammonia, etc.). H₂ and O₂ mayalso be used for medical, industrial, and/or other scientific processes(e.g., as medical grade oxygen, combustion with acetylene in anoxy-acetylene torch for welding and cutting metals, etc.). Those ofordinary skill in the art will be aware of uses for O₂ and/or H₂. Othernon-limiting examples of device uses include O₂ production (e.g.,gaseous oxygen), H₂ production (e.g., gaseous hydrogen), H₂O₂production, ammonia oxidation, hydrocarbon (e.g., methanol, methane,ethanol, and the like) oxidation, exhaust treatment, etc.

In some embodiments, a photoelectrochemical cell is provided that allowsfor electrochemically producing oxygen and/or hydrogen gases from waterand systems and/or methods associated with the same. In someembodiments, the photoelectrochemical cell may comprise a photoanode(e.g., comprising a photoactive electrode and a catalytic material,wherein a catalytic material is integrally connected with thephotoactive electrode (or photosensitizing agent)) and an electrode (orphotocathode). The catalytic material may comprise metal ionic speciesand anionic species and/or may not consist essentially of metal oxide ormetal hydroxide. Illumination of the device (e.g., by exposure toelectromagnetic radiation) may produce oxygen gas. In some instances,hydrogen gas may also be produced at the electrode. As shown in FIG. 1,in a non-limiting configuration, a device comprises a chamber 128, aphotoactive electrode 130, an electrode (or second photoactiveelectrode) 134, wherein the photoactive electrode is biased positivelywith respect to the electrode, means for connecting the photoactiveelectrode and the electrode 131, an electrolyte 132, wherein thephotoactive electrode and the electrode are in fluid contact with theelectrolyte, and in most cases, a power source 138 in electricalcommunication with the photoactive electrode and the electrode. In somecases, the device may also comprise a resistor 136.

A photoactive electrode biased negatively or positively towards anelectrode (or second photoactive electrode) means that the potential ofthe photoactive electrode is negative or positive with respect to thepotential of the electrode (or second photoactive electrode). Theelectrode may be biased negatively or positively with respect to thephotoactive electrode by less than about 1.23 V (e.g., the minimumdefined by the thermodynamics of transforming water into oxygen andhydrogen gas), less than about 1.3 V, less than about 1.4 V, less thanabout 1.5 V, less than about 1.6 V, less than about 1.7 V, less thanabout 1.8 V, less than about 2 V, less than about 2.5 V, and the like.In some cases, the bias may be between about 1.5 V and about 2.0 V,about 1.6 V and about 1.9 V, or is about 1.6 V, between about 1 V andabout 2.5 V, between about 1.5 and about 2.5 V, and the like. Voltagemay be applied to the photoactive electrode (e.g., via an external powersource and/or by exposing the photoactive electrode to light) togenerate electron-hole pairs. The electron-hole pairs may be separatedbetween the photoactive electrode and the electrode, to producephotoelectrochemical reduction and photoelectrochemical oxidationreactions at the electrode and photoactive electrode, respectively,thereby producing oxygen gas. In the case of the photoactive electrode,holes combine with water molecules (H₂O) to produce an oxidationreaction, thereby producing oxygen gas. The reverse reaction may occurat the electrode, where electrons combine with protons (e.g., H⁺, or aproton source), to produce a reduction reaction, thereby producinghydrogen gas. The net effect is a flow of electrons from the firstphotoactive electrode to the second electrode, resulting in reduction atthe latter (hydrogen gas formation), and oxidation at the former (oxygengas formation). In some cases, the hydrogen and/or oxygen gases producedmay be stored and used in further reactions.

As another non-limiting embodiment, in some cases, thephotoelectrochemical cell may comprise a hybridphotovoltaic/photoelectrode. A hybrid photovoltaic/photoelectrodegenerally comprises a photoelectrode that is electrolytically active(e.g., an electrode where water oxidation takes place), a photovoltaiccell, which acts to provide a voltage bias to the photoelectrode, and aelectrode (e.g., where the corresponding reduction of protons may occurto fulfill the second half-reaction in overall water splitting for thedevice.). A non-limiting illustration of a photoelectrochemical cell isshown in FIG. 10. In this figure, the hybrid photovoltaic/photoelectrodecomprises photoanode 202 in electrical connection with p-n junctionsolar cell 204 (e.g., comprising silicon), electrode 206, and in somecases, coating 200 to protect the solar cell and electrode from outsideexposure (e.g., to the electrolyte, etc.). Upon exposure to light, thephotoanode absorbs photons having energy equal to or greater than itsband gap while the rest of the light is transmitted to the solar cell.The solar cell provides the additional energy required to bias thedevice for water electrolysis.

A non-limiting example of a photoelectrochemical cell is depicted inFIG. 11. The photoelectrochemical cell comprises housing 298, in whichat least one section or side of the housing is substantially transparentto light (e.g., wall 298 a and walls 298). During operation, thephotoelectrochemical cell may be illuminated on the wall(s) which aresubstantially transparent. The housing may comprise at least firstoutlet 320 and second outlet 322 for the collection of O₂ and H₂ gases,respectively, produced during the photoelectrochemical reaction. Thehousing may comprise at least one photovoltaic cell comprising firstelectrode (or photoanode) 306, and second electrode (or photocathode)302. In some cases, material 304 may be present between the firstelectrode and the second electrode (e.g., a non-doped semiconductor).The cell also comprises an electrolyte (e.g., 300, 318). The cell alsomay comprise material 316. Material 316 may be a porous electricallyconductive material (e.g., valve metal, metallic compound) wherein theelectrolyte (e.g., 318) fills the pores of the material. In someembodiments, a catalytic material 308 may associate with material 316(e.g., indirect association) as compared to direct association with thephotoactive electrode (or electrode). Without wishing to be bound bytheory, material 316 may act as a membrane and may allow for thetransmission of electrons generated at first electrode (or photoactiveelectrode) 306 to outer surface 324 of material 316. Material 316 mayalso be selected such that no oxygen gas is produced in the pores ofmaterial 316, for example, if the overpotential for production of oxygengas is high. Oxygen gas may form on or near surface 324 of material 316(e.g., via the composition associated with outer surface 324 or material316). Non-limiting examples of materials which may be suitable for useas material 316 includes titanium, zirconium, vanadium, hafnium,niobium, tantalum, tungsten, or alloys thereof. In some cases, thematerial may be a valve metal nitride, carbide, borides, etc., forexample, titanium nitride, titanium carbide, or titanium boride. In somecases, the material may be titanium oxide, or doped titanium oxide(e.g., with niobium, tantalum, tungsten, fluorine, etc.).

In some cases, a photoelectrochemical cell may be abi-photoelectrochemical device or tandem photoelectrochemical cell andmay comprise a first and a second photoelectrode. The first and secondphotoelectrodes may work in tandem to split water to produce hydrogenand oxygen gases using electromagnetic radiation (e.g., visible light,solar energy). The first and the second photoelectrodes may be inelectrical communication with one another. A non-limiting arrangement ofa bi-photoelectrochemical cell is shown in FIG. 12. In this figure,150-1 and 151 are transparent materials (e.g., glass) through whichlight can pass. The light may pass through material 150-1 and throughelectrolyte 152 (e.g., aqueous electrolyte) and impinge on aphotoelectrode comprising components 153 (e.g., light absorbingmaterial, catalytic material, etc.) and 154-1 (e.g., material which maycollect electrons produced by light absorbing material, catalyticmaterial, etc.). In some cases, in this device, photoelectrode 153/154-1may absorb only a part of the visible light spectrum (e.g., blue andgreen light) and the remainder of the spectrum (e.g., red and yellowlight) may pass through another transparent material (e.g., glass,150-2) to a second cell. Oxygen gas may be produced at photoelectrode153/154-1. The second cell may comprise material 154-2 (e.g., aconducting oxide material) and material 156 (e.g., a dye-derivatizedmetal oxide material), which may function as a light-driven electricbias and may increase the electrochemical potential of the electronswhich emerge from photoelectrode 153/154-1. The second cell may alsocomprise electrolyte 157 (e.g., organic redox electrolyte) and counterelectrode 158. Behind counter electrode 158, there may also be acompartment comprising electrolyte 159, in which hydrogen gas may beproduced at cathode 160. Electrolytes 152 and 159 may be substantiallysimilar, in some embodiments, and may be connected by a ion-conductingmembrane or glass frit 161.

As another example, as shown in FIG. 13, a bi-photoelectrochemical cellmay comprise first photoelectrode 180 (e.g., comprising a photoanode asdescribed herein), second photoelectrode 182 biased negatively withrespect to the first photoelectrode (e.g., photocathode such as p-typeGaP), electrolyte 190 (e.g., an aqueous electrolyte), and means forconnecting 184 the first and the second photoelectrode. In some cases,the bi-photoelectrochemical cell may optionally comprise power source188 (e.g., especially in cases where the photoanode and the photocathodecomprise similar materials but are differently doped such as p-type andn-type TiO₂) and/or a resistor 186.

Yet another embodiment for a photoelectrochemical cell for theelectrolysis of water, may comprise a container, an aqueous electrolytein the container, wherein the pH of the electrolyte is neutral or below,a photoanode mounted in the container and in contact with theelectrolyte, wherein the first electrode comprises a photoactiveelectrode, metal ionic species and anionic species, the metal ionicspecies and the anionic species defining a substantially non-crystallinecomposition and have an equilibrium constant, K_(sp), between about 10⁻³and 10⁻¹⁰ when the metal ionic species is in an oxidation state of (n)and have a K_(sp) less than about 10⁻¹⁰ when the metal ionic species isin an oxidation state of (n+x), an electrode (or second photoactive)mounted in the container and in contact with the electrolyte, whereinthe electrode is biased negatively with respect to the photoanode, andmeans for connecting the photoanode and the electrode. In thisembodiment, when a voltage is applied between the photoanode and theelectrode, gaseous hydrogen may be evolved at the electrode and gaseousoxygen may be produced at the photoanode.

The performance of a photoanode of a device may be measured by currentdensity (e.g., geometric and/or total current density), wherein thecurrent density is a measure of the density of flow of a conservedcharge. For example, the current density is the electric current perunit area of cross section. In some cases, the current density (e.g.,geometric current density and/or total current density, as describedherein) of a photoanode as described herein is greater than about 0.1mA/cm², greater than about 1 mA/cm², greater than about 5 mA/cm²,greater than about 10 mA/cm², greater than about 20 mA/cm², greater thanabout 25 mA/cm², greater than about 30 mA/cm², greater than about 50mA/cm², greater than about 100 mA/cm², greater than about 200 mA/cm²,and the like.

In some embodiments, the current density can be described as thegeometric current density. The geometric current density, as usedherein, is current divided by the external surface area of thephotoanode. The external surface area of a photoanode will be understoodby those of ordinary skill in the art and refers to the surface definingthe outer boundary of the photoanode, for example, the area that may bemeasured by a macroscopic measuring tool (e.g., a ruler) and does notinclude the internal surface area (e.g., area within pores of a porousmaterial such as a foam, or surface area of those fibers of a mesh thatare contained within the mesh and do not define the outer boundary,etc.).

In some cases, the current density can be described as the total currentdensity. Total current density, as used herein, is the current densitydivided by essentially the total surface area (e.g., the total surfacearea including all pores, fibers etc.) of the photoanode. In some cases,the total current density may be approximately equal to the geometriccurrent density (e.g., in cases where the photoanode is not porous andthe total surface area is approximately equal to the geometric surfacearea).

In some embodiments, a device and/or photoanode as described herein iscapable of producing at least about 1 umol (micromole), at least about 5umol, at least about 10 umol, at least about 20 umol, at least about 50umol, at least about 100 umol, at least about 200 umol, at least about500 umol, at least about 1000 umol oxygen and/or hydrogen, or more, percm² at the photoanode at which oxygen production and/or hydrogenproduction occurs, respectively, per hour. The area of the photoanodemay be the geometric surface area or the total surface area, asdescribed herein.

The devices and methods as described herein, in some cases, may proceedat about ambient conditions. Ambient conditions define the temperatureand pressure relating to the device and/or method. For example, ambientconditions may be defined by a temperature of about 25° C. and apressure of about 1.0 atmosphere (e.g., 1 atm, 14 psi). In some cases,the conditions may be essentially ambient. Non-limiting examples ofessentially ambient temperature ranges include between about 0° C. andabout 40° C., between about 5° C. and about 35° C., between about 10° C.and about 30° C., between about 15° C. and about 25° C., at about 20°C., at about 25° C., and the like. Non-limiting examples of essentiallyambient pressure ranges include between about 0.5 atm and about 1.5 atm,between about 0.7 atm and about 1.3 atm, between about 0.8 and about 1.2atm, between about 0.9 atm and about 1.1 atm, and the like. In aparticular case, the pressure may be about 1.0 atm. Ambient oressentially ambient conditions can be used in conjunction with any ofthe devices, compositions, catalytic materials, and/or methods describedherein, in conjunction with any conditions (for example, conditions ofpH, etc.).

In some cases, the devices and/or methods as described herein mayproceed at temperatures above ambient temperature. For example, a deviceand/or method may be operated at temperatures greater than about 30° C.,greater than about 40° C., greater than about 50° C., greater than about60° C., greater than about 70° C., greater than about 80° C., greaterthan about 90° C., greater than about 100° C., greater than about 120°C., greater than about 150° C., greater than about 200° C., or greater.Efficiencies can be increased, in some instances, at temperatures higherthan ambient. The temperature of the device may be selected such thatthe water provided and/or formed is in a gaseous state (e.g., attemperatures greater than about 100° C.). In other cases, devices and/ormethods as described herein may proceed at temperatures below ambienttemperature. For example, a device and/or method may be operated attemperatures less than about 20° C., less than about 10° C., less thanabout 0° C., less than about −10° C., less than about −20° C., less thanabout −30° C., less than about −40° C., less than about −50° C., lessthan about −60° C., less than about −70° C. or the like. In someinstances, the temperature of the device and/or method may be affectedby an external temperature source (e.g., a heating and/or cooling coil,infrared light, refrigeration, etc.). In other instances, however, thetemperature of the device and/or method may be affected by internalprocesses, for example, exothermic and/or endothermic reactions, etc. Insome cases, the device and/or method may be operated at approximatelythe same temperature throughout the use of the device and/or method. Inother cases, the temperature may be changed at least once and/orgradually during the use of the device and/or method. In a particularembodiment, the temperature of the device may be elevated during timeswhen the device is used in conjugation with sunlight or other radiativepower sources.

In some embodiments, the water provided and/or formed during use of amethod and/or device as described herein may be in a gaseous state(e.g., steam). Those of ordinary skill in the art can apply knownelectrochemical techniques carried out with steam, in some cases,without undue experimentation. As an exemplary embodiment, water may beprovided in a gaseous state to an electrochemical device (e.g.,high-temperature electrolysis or steam electrolysis) comprising aphotoanode. In some cases, the gaseous water may be produced by a deviceor system which inherently produces steam (e.g., a nuclear power plant).Without wishing to be bound by theory, in some cases, providing water ina gaseous state may allow for the electrolysis to proceed moreefficiently as compared to a similar device when provided water in aliquid state. This may be due to the higher input energy of the watervapor. In some instances, the gaseous water provided may comprise othergases (e.g., hydrogen gas, nitrogen gas, etc.).

Individual aspects of the overall electrochemistry and/or chemistryinvolved in electrochemical devices such as those described herein aregenerally known, and not all will be described in detail herein. It isto be understood that the specific electrochemical devices describedherein are exemplary only, and the components, connections, andtechniques as described herein can be applied to virtually any suitableelectrochemical device including those with a variety of solid, liquid,and/or gaseous fuels, and a variety of photoanodes, electrodes,photocathodes, and/or electrolytes, which may be liquid or solid underoperating conditions (where feasible; generally, for adjacent componentsone will be solid and one will be liquid if any are liquids). It is alsoto be understood that photoelectrochemical device unit arrangementsdiscussed are merely examples of photoelectrochemical devices that canmake use of photoanodes as described herein. Many structuralarrangements other than those disclosed herein, which make use of andare enabled by the present invention, will be apparent to those ofordinary skill in the art.

A photoelectrochemical device accordingly may be combined withadditional electrochemical devices (e.g., a fuel cell, an electrolyticdevice, etc.) to form a larger device or system. In some embodiments,this may take the form of a stack of units or devices. Where more thanone electrochemical device is combined, the devices may all be devicesaccording to an embodiment the present invention, or one or more devicesaccording to an embodiment the present invention may be combined withother photoelectrochemical devices, such as a fuel cell. It is to beunderstood that where this terminology is used, any suitableelectrochemical device, which those of ordinary skill in the art wouldrecognize could function in accordance with the systems and techniquesas described herein can be substituted.

Water may be provided to the systems, devices, photoanodes, and/or forthe methods provided herein, using any suitable source. In some cases,the water is provided from a substantially pure water source (e.g.,distilled water, deionized water, chemical grade water, etc.). In somecases, the water may be bottled water. In some cases, the water isprovided from a natural and/or impure water source (e.g., tap water,lake water, ocean water, rain water, lake water, pond water, sea water,potable water, brackish water, industrial process water, etc.). In somecases, although it need not be, the water is not purified prior to use(e.g., before being provided to the system/photoanode for electrolysis).In some instances, the water may be filtered to remove particulatesand/or other impurities prior to use. In some embodiments, the waterthat is electrolyzed to produce oxygen gas (e.g., using a photoanodeand/or device as described here) may be substantially pure. The purityof the water may be determined using one or more methods known to thoseof ordinary skill in the art, for example, resistivity, carbon content(e.g., through use of a total organic carbon analyzer), UV absorbance,oxygen-absorbance test, limulus ameobocyte lysate test, etc. In someembodiments, the water may contain at least one impurity. In someembodiments, the at least one impurity may be substantiallynon-participative in the catalytic reaction. That is, the at least oneimpurity does not participate in aspects of the catalytic cycle and/orregeneration mechanism. The at least one impurity may be solid (e.g.,particulate matter), a liquid, and/or a gas. In some cases, the impuritymay be solubilized and/or dissolved. For example, an impurity maycomprise ionic species. In some cases, an impurity may be an impuritywhich may generally be present in a water source (e.g., tap water,non-potable water, potable water, sea water, etc.). In a particularembodiment, the water source may be sea water and one of the impuritiesmay be chloride ions, as described herein. In some cases, an impuritymay comprise a metal such as a metal element (including heavy metals), ametal ion, a compound comprising at least one metal, an ionic speciescomprising a metal, etc. For example, an impurity comprising metal maycomprise an alkaline earth metal, an alkali metal, a transition metal,or the like. Specific non-limiting examples of metals include lithium,sodium, magnesium, titanium, vanadium, chromium, manganese, iron,cobalt, nickel, copper, zinc, potassium, mercury, lead, barium, etc. Insome instances, an impurity comprising a metal may be the same ordifferent than the metal comprised in the metal ionic species of acatalytic material as described herein. In some cases, the impurity maycomprise organic materials, for example, small organic molecules (e.g.,bisphenol A, trimethylbenzene, dioxane, nitrophenol, etc.),microorganisms (such as bacteria (e.g., E. coli, coliform, etc.),microbes, fungi, algae, etc.), other biological materials,pharmaceutical compounds (e.g., drugs, decomposition products fromdrugs), herbicides, pyrogens, pesticides, proteins, radioactivecompounds, inorganic compounds (e.g., compounds comprising boron,silicon, sulfur, nitrogen, cyanide, phosphorus, arsenic, sodium, etc.;carbon dioxide, silicates (e.g., H₄SiO₄), ferrous and ferric ironcompounds, chlorides, aluminum, phosphates, nitrates, etc.), dissolvedgases, suspended particles (e.g., colloids), or the like. In some cases,an impurity may be a gas, for example, carbon monoxide, ammonia, carbondioxide, oxygen gas, and/or hydrogen gas. In some cases, the gasimpurity may be dissolved in the water. In some cases, a photoanode maybe capable of operating at approximately the same, at greater than about95%, at greater than about 90%, at greater than about 80%, at greaterthan about 70%, at greater than about 60%, at greater than about 50%, orthe like, of the activity level using water containing at least oneimpurity versus the activity using water that does not substantiallycontain the impurity under essentially identical conditions. In somecases, a photoanode may catalytically produce oxygen from watercontaining at least one impurity such that less than about 5 mol %, lessthan about 3 mol %, less than about 2 mol %, less than about 1 mol %,less than about 0.5 mol %, less than about 0.1 mol %, less than about0.01 mol % of the products produced comprise any portion of the at leastone impurity.

In some cases, an impurity may be present in the water in an amountgreater than about 1 ppt, greater than about 10 ppt, greater than about100 ppt, greater than about 1 ppb, greater than about 10 ppb, greaterthan about 100 ppb, greater than about 1 ppm, greater than about 10 ppm,greater than about 100 ppm, greater than about 1000 ppm, or greater. Inother cases, an impurity may be present in the water in an amount lessthan about 1000 ppm, less than about 100 ppm, less than about 10 ppm,less than about 1 ppm, less than about 100 ppb, less than about 10 ppb,less than about 1 ppb, less than about 100 ppt, less than about 10 ppt,less than about 1 ppt, or the like. In some cases, the water may containat least one impurity, at least two impurities, at least threeimpurities, at least five impurities, at least ten impurities, at leastfifteen impurities, at least twenty impurities, or greater. In somecases, the amount of impurity may increase or decrease during operationof the photoanode and/or device. That is, an impurity may be formedduring use of the photoanode and/or device. For example, in some cases,the impurity may be a gas (e.g., oxygen gas and/or hydrogen gas) formedduring the electrolysis of water. Thus, in some cases, the water maycontain less than about 1000 ppm, less than about 100 ppm, less thanabout 10 ppm, less than about 1 ppm, less than about 100 ppb, less thanabout 10 ppb, less than about 1 ppb, less than about 100 ppt, less thanabout 10 ppt, less than about 1 ppt, or the like, prior to operation ofthe photoanode and/or device.

In some embodiments, the at least one impurity may be an ionic species.In some cases, when the water contains at least one ionic species, thewater purity may be determined, at least in part, by measuring theresistivity of the water. The theoretical resistivity of water at 25° C.is about 18.2 MΩ·cm. The resistivity of water that is not substantiallypure may be less than about 18 MΩ·cm, less than about 17 MΩ·cm, lessthan about 16 MΩ·cm, less than about 15 MΩ·cm, less than about 12 MΩ·cm,less than about 10 MΩ·cm, less than about 5 MΩ·cm, less than about 3MΩ·cm, less than about 2 MΩ·cm, less than about 1 MΩ·cm, less than about0.5 MΩ·cm, less than about 0.1 MΩ·cm, less than about 0.01 MΩ·cm, lessthan about 1000 Ω·cm, less than about 500 Ω·cm, less than about 100Ω·cm, less than about 10 Ω·cm, or less. In some cases, the resistivityof the water may be between about 10 MΩ·cm and about 1 Ω·cm, betweenabout 1 MΩ·cm and about 10 Ω·cm, between about 0.1 MΩ·cm and about 100Ω·cm, between about 0.01 MΩ·cm and about 1000 Ω·cm, between about 10,000Ω·cm and about 1,000 Ω·cm, between about 10,000 Ω·cm and about 100 Ω·cm,between about 1,000 and about 1 Ω·cm, between about 1,000 and about 10Ω·cm, and the like. In some cases, when the water source is tap water,the resistivity of the water may be between about 10,000 Ω·cm and about1,000 Ω·cm. In some cases, when the water source is sea water, theresistivity of the water may be between about 1,000 Ω·cm and about 10Ω·cm. In some instances, where the water may be taken from an impuresource and purified prior to use, the water may be purified in a mannerwhich does not resistivity of the water by a factor of more than about5%, about 10%, about 20%, about 25%, about 30%, about 50%, or the like.Those of ordinary skill in the art will be aware of methods to determinethe resistivity of water.

In some cases, where the water is obtained from an impure water sourceand/or has a resistivity of less than about 16 MΩ·cm the water may bepurified (e.g., filtered) in a manner that changes its resistivity by afactor of less than about 50%, less than about 30%, less than about 25%,less than about 20%, less than about 15%, less than about 10%, less thanabout 5%, or less, after being drawn from the source prior to use in theelectrolysis.

In some embodiments, the water may contain halide ions (e.g., fluoride,chloride, bromide, iodide), for example, such that a photoanode may beused for the desalination of sea water. In some cases, the halide ionsmight not be oxidized (e.g., to form halogen gas such as Cl₂) during thecatalytic production of oxygen from water. Without wishing to be boundby theory, halide ions (or other anionic species) that might not beincorporated in the catalytic material (e.g., within the lattice of thecatalytic material) might not be oxidized during the catalytic formationof oxygen from water. This may be because the halide ions might notreadily form bonds with the metal ionic species, and therefore, may onlyhave access to outer sphere mechanism for oxidation. In some instances,oxidation of halide ions by an outer sphere mechanism may be notkinetically favorable. In some cases, a photoanode may catalyticallyproduce oxygen from water comprising halide ions such that less thanabout 5 mol %, less than about 3 mol %, less than about 2 mol %, lessthan about 1 mol %, less than about 0.5 mol %, less than about 0.1 mol%, less than about 0.01 mol % of the gases evolved comprise oxidizedhalide species. In some embodiments, the impurity is sodium chloride.

In some cases, under catalytic condition, halide ions (or otherimpurities) might not associate with a catalytic material and/or withmetal ionic species. In some instances, a complex comprising a halideion and a metal ionic species may be substantially soluble such that thecomplex does not form a catalytic material and/or associate with thephotoactive electrode and/or photoanode. In some cases, the catalyticmaterial may comprise less than about 5 mol %, less than about 3 mol %,less than about 2 mol %, less than about 1 mol %, less than about 0.5mol %, less than about 0.1 mol %, less than about 0.01 mol % of thehalide ion impurities.

In some cases, the rate of oxidation of water may dominate over the rateof oxidation of halide ions (or other impurities) due to various factorsincluding thermodynamics, solubility, and the like. For example, thebinding affinity of a metal ionic species for an anionic species may besubstantially greater than the binding affinity of the metal ionicspecies for a halide ion, such that the coordination sphere of the metalionic species may be substantially occupied by the anionic species. Inother cases, the halide ions might not be incorporated into the latticeof a catalytic material (e.g., as part of the lattice or within theinterstitial holes of the lattice) due to the size of the halide ion(e.g., the halide is too large or too small to be incorporated into thelattice of the catalytic material). Those of ordinary skill in the artwill be able to determine if a photoanode as described herein is able tocatalytically produce oxygen using water containing halide ions, forexample, by monitoring the production of halogen gas (or speciescomprising oxidized halide ions) using suitable techniques, for example,mass spectrometry.

Various components of the invention, such as the photoanode, electrode,photocathode, power source, electrolyte, separator, container,circuitry, insulating material, gate electrode, etc. can be fabricatedby those of ordinary skill in the art from any of a variety ofcomponents, as well as those described in any of those patentapplications described herein. Components of the invention can bemolded, machined, extruded, pressed, isopressed, infiltrated, coated, ingreen or fired states, or formed by any other suitable technique. Thoseof ordinary skill in the art are readily aware of techniques for formingcomponents of devices herein. In some cases, components (e.g.,photoanodes, electrodes, electrolyte, electrical connectors, wires,etc.) of a device may be selected as to minimize the ohmic resistancesof the device. This may aid in achieving the maximum energy conversionefficiency possible for a selected device.

While electromagnetic radiation sources are described herein, it shouldbe understood that electromagnetic radiation may be provided in anysuitable arrangement or using any suitable source, and may depend on thearrangement and components of the photoelectrochemical device. In somecases, electromagnetic radiation may be provided to one or more surfaceand/or components of a photoelectrochemical device. For example,electromagnetic radiation may be provided directly to a catalyticmaterial (e.g., light is shone on the catalytic material), or may beprovided indirectly, for example, through the backside of the catalyticmaterial (e.g., light is shone through one or more other materials,including, but not limited to, the photoactive electrode). Those ofordinary skill in the art will be able to determine the portions of adevice to be exposed to electromagnetic radiation.

In some cases, the device may comprise a light management system and/orsolar concentrator, which are capable of focusing electromagneticradiation and/or solar energy. Generally, light management systems orsolar concentrators may receive electromagnetic radiation and/or solarenergy over a first surface area and direct the received radiation to asecond, smaller, surface area. Light management systems and solarconcentrators will be known to those of ordinary skill in the art andmay comprise, for example, magnifying lenses, parabolic mirrors, and/orFresnel lenses for focusing incoming light and/or solar energy. In somecases, the light management system or solar collector may collect andwaveguide the light to an area or surface of the photoelectrochemicaldevice, for example, a surface associated with the catalytic material, aphotoactive electrode, a photoanode, a photocathode, etc.

In some cases, a device may be portable. That is, the device may be ofsuch size that it is small enough that it is movable. In someembodiments, a device of the present invention is portable and can beemployed at or near a desired location (e.g., water supply location,field location, etc.). For example, the device may be transported and/orstored at a specific location. In some case, the device may be equippedwith straps or other components (e.g., wheels) such that the device maybe carried or transported from a first location to a second location.Those of ordinary skill in the art will be able to identify a portabledevice. For instance, the portable device may have a weight less thanabout 25 kg, less than about 20 kg, less than about 15 kg, less thanabout 1 kg, less than about 8 kg, less than about 7 kg, less than about6 kg, less than about 5 kg, less than about 4 kg, less than about 3 kg,less than about 2 kg, less than about 1 kg, and the like, and/or have alargest dimension that is no more than 50 cm, less than about 40 cm,less than about 30 cm, less than about 20 cm, less than about 10 cm, andthe like. The weight and/or dimensions of the device typically may ormight not include components associated with the device (e.g., watersource, water source reservoir, oxygen and/or hydrogen storagecontainers, etc.).

An electrolyte, as known to those of ordinary skill in the art is anysubstance containing free ions that is capable of functioning as anionically conductive medium. In some cases, an electrolyte may comprisewater, which may act as the water source. The electrolyte may be aliquid, a gel, and/or solid. The electrolyte may also comprise methanol,ethanol, sulfuric acid, methanesulfonic acid, nitric acid, mixtures ofHCl, organic acids like acetic acid, etc. In some cases, the electrolytecomprises mixtures of solvents, such as water, organic solvents, aminesand the like. In some cases, the pH of the electrolyte may be aboutneutral. That is, the pH of the electrolyte may be between about 5.5 andabout 8.5, between about 6.0 and about 8.0, about 6.5 about 7.5, and/orthe pH is about 7.0. In a particular case, the pH is about 7.0. In othercases, the pH of the electrolyte is about neutral or acidic. In thesecases, the pH may range from about 0 to about 8, about 1 to about 8,about 2 to about 8, about 3 to about 8, about 4 to about 8, about 5 toabout 8, about 0 to about 7.5, about 1 to about 7.5, about 2 to about7.5, about 3 to about 7.5, about 4 to about 7.5, about 5 to about 7.5.In yet other cases, the pH may be between about 6 and about 10, about 6and about 11, about 7 and about 14, about 2 and about 12, and the like.In a specific embodiment, the pH is between about 6 and about 8, betweenabout 5.5 and about 8.5, between about 5.5 and about 9.5, between about5 and about 9, between about 3 and about 11, between about 4 and about10, or any other combination thereof. In some cases, when theelectrolyte is a solid, the electrolyte may comprise a solid polymerelectrolyte. The solid polymer electrolyte may serve as a solidelectrolyte that conducts protons and separate the gases produces and orutilized in the electrochemical cell. Non-limiting examples of a solidpolymer electrolyte are polyethylene oxide, polyacrylonitrile, andcommercially available NAFION.

In some cases, the electrolyte is used to selectively transport one ormore ionic species. In some embodiments, the electrolyte(s) are at leastone of oxygen ion conducting membranes, proton conductors, carbonate(CO₃ ⁻²) conductors, OH⁻ conductors, and/or mixtures thereof. In somecases, the electrolyte(s) are at least one of cubic fluorite structures,doped cubic fluorites, proton-exchange polymers, proton-exchangeceramics, and mixtures thereof. Further, oxygen-ion conducting oxidesthat may be used as the electrolyte(s) include doped ceria compoundssuch as gadolinium-doped ceria (Gd_(1-x)Ce_(x)Ce_(x)O_(2-d)) orsamarium-doped ceria (Sm_(1-x)Ce_(x)O_(2-d)), doped zirconia compoundssuch as yttrium-doped zirconia (Y_(1-x)Zr_(x)O_(2-d)) or scandium-dopedzirconia (Sc_(1-x)Zr_(x)O_(2-d)), perovskite materials such asLa_(1-x)Sr_(x)Ga_(1-y)Mg_(y)O_(3-d), yttria-stabilized bismuth oxide,and/or mixtures thereof. Examples of proton conducting oxides that maybe used as electrolyte(s) include, but are not limited to, undoped andyttrium-doped BaZrO_(3-d), BaCeO_(3-d), and SrCeO_(3-d) as well asLa_(1-x)Sr_(x)NbO_(3-d).

In some embodiments, the electrolyte may comprise an ionicallyconductive material. In some embodiments, the ionically conductivematerial may comprise the anionic species comprised in the catalyticmaterial on at least one photoanode. The presence of the anionic speciesin the electrolyte, during use of the photoanode comprising a catalyticmaterial, may shift the dynamic equilibrium towards the association ofthe anionic species and/or metal ionic species with the photoanode, asdescribed herein. Non-limiting examples of other ionically conductivematerials include metal oxy-compounds, soluble inorganic and/or organicsalts (e.g., sodium or potassium chloride, sodium sulfate, quaternaryammonium hydroxides, etc.).

In some cases, the electrolyte may comprise additives. For example, theadditive may be an anionic species (e.g., as comprised in the catalyticmaterial associated with a photoactive electrode). For example, aphotoanode used in a device may comprise a photoactive electrode and acatalytic material comprising at least one anionic species and at leastone metal ionic species. The electrolyte may comprise the at least oneanionic species. In some cases, the electrolyte can comprise an anionicspecies which is different from the at least one anionic speciescomprised in the catalytic material. For example, the catalytic materialmay comprise phosphate anions and the electrolyte may comprise borateanions. In some cases, when the additive is an anionic species, theelectrolyte may comprise counter cations (e.g., when the anionic speciesis added as a complex, a salt, etc.). The anionic species may be goodproton-accepting species. In some cases, the additive may be a goodproton-accepting species which is not anionic (e.g., is a neutral base).Non-limiting examples of good proton-accepting species which are neutralinclude pyridine, imidazole, and the like.

In some cases, the electrolyte may be recirculated in theelectrochemical device. That is, a device may be provided which is ableto move the electrolyte in the electrochemical device. Movement of theelectrolyte in the electrochemical device may help decrease the boundarylayer of the electrolyte. The boundary layer is the layer of fluid inthe immediate vicinity of an electrode and/or photoanode. In general,the extent to which a boundary layer exists is a function of the flowvelocity of the liquid in a solution. Therefore, if the fluid isstagnant, the boundary layer may be much larger than if the fluid wasflowing. Therefore, movement of the electrolyte in thephotoelectrochemical device may decrease the boundary layer and improvethe efficiency of the device.

In most embodiments, a device may comprise at least one photoanode asdescribed herein (e.g., comprising a photoactive electrode and acatalytic material). In some instances, the device can additionallycomprise at least one electrode and/or photocathode. In general, anelectrode may be any material that is substantially electricallyconductive. The electrode may be transparent, semi-transparent,semi-opaque, and/or opaque. The electrode may be a solid, semi-porous orporous. Non-limiting examples of electrodes include indium tin oxide(ITO), fluorine tin oxide (FTO), glassy carbon, metals,lithium-containing compounds, metal oxides (e.g., platinum oxide, nickeloxide), graphite, nickel mesh, carbon mesh, and the like. Non-limitingexamples of suitable metals include gold, copper, silver, platinum,nickel, cadmium, tin, and the like. In some instances, the electrode maycomprise nickel (e.g., nickel foam or nickel mesh). Nickel foam andnickel mesh materials will be known to those of ordinary skill in theart and may be purchase from commercial sources. Nickel mesh usuallyrefers to woven nickel fibers. Nickel foam generally refers to amaterial of non-trivial thickness (e.g., about 2 mm) comprising aplurality of holes and/or pores. In some cases, nickel foam may be anopen-cell, metallic structure based on the structure of an open-cellpolymer foam, wherein nickel metal is coated onto the polymer foam. Theelectrodes may also be any other metals and/or non-metals known to thoseof ordinary skill in the art as conductive (e.g., ceramics). Theelectrodes may also be photoactive electrodes used inphotoelectrochemical cells. The electrode may be of any size or shape.Non-limiting examples of shapes include sheets, cubes, cylinders, hollowtubes, spheres, and the like. The electrode may be of any size.Additionally, the electrode may comprise a means to connect theelectrode and to another electrode, a power source and/or anotherelectrical device.

Various electrical components of device may be in electricalcommunication with at least one other electrical component by a meansfor connecting. A means for connecting may be any material that allowsthe flow of electricity to occur between a first component and a secondcomponent. A non-limiting example of a means for connecting twoelectrical components is a wire comprising a conductive material (e.g.,copper, silver, etc.). In some cases, the device may also compriseelectrical connectors between two or more components (e.g., a wire andan electrode and/or photoanode). In some cases, a wire, electricalconnector, or other means for connecting may be selected such that theresistance of the material is low. In some cases, the resistances may besubstantially less than the resistance of the electrodes, photoanodes,and/or electrolyte of the device.

In some embodiments, a power source may be provided to supply DC or ACvoltage to an electrochemical device. Non-limiting examples includebatteries, power grids, regenerative power supplies (e.g., wind powergenerators, photovoltaic cells, tidal energy generators), generators,and the like. The power source may comprise one or more of such powersupplies (e.g., batteries and a photovoltaic cell).

In some embodiment, a device may comprise a power management system,which may be any suitable controller device, such as a computer ormicroprocessor, and may contain logic circuitry which decides how toroute the power streams. The power management system may be able todirect the energy provided from a power source or the energy produced bythe electrochemical device to the end point, for example, anotherdevice. It is also possible to feed electrical energy to a power sourceand/or to consumer devices (e.g., cellular phone, television).

In some cases, electrochemical devices may comprise a separatingmembrane. The separating membranes or separators for thephotoelectrochemical device may be made of suitable material, forexample, a plastic film. Non-limiting examples of plastic films includedinclude polyamide, polyolefin resins, polyester resins, polyurethaneresin, or acrylic resin and containing lithium carbonate, or potassiumhydroxide, or sodium-potassium peroxide dispersed therein.

A container may be any receptacle, such as a carton, can, or jar, inwhich components of an electrochemical device may be held or carried. Acontainer may be fabricated using any known techniques or materials, aswill be known to those of ordinary skill in the art. For example, insome instances, the container may be fabricated from gas, polymer,metal, and the like. The container may have any shape or size, providingit can contain the components of the electrochemical device. Componentsof the electrochemical device may be mounted in the container. That is,a component (e.g., an electrode) may be associated with the containersuch that it is immobilized with respect to the container, and in somecases, is supported by the container. A component may be mounted to thecontainer using any common method and/or material known to those skilledin the art (e.g., screws, wires, adhesive, etc). The component may ormight not physically contact the container. In some cases, an electrodemay be mounted in the container such that the electrode is not incontact with the container, but is mounted in the container such that itis suspended in the container.

Where the catalytic material, photoanode, and/or electrode of theinvention is used in connection with an electrochemical device such as afuel cell, any suitable fuels, oxidizers, and/or reactant product may beprovided to and/or produced by electrochemical devices. In someembodiments, the photoelectrochemical device may produce a fuel (e.g.,hydrogen). In a particular embodiment, in addition to oxygen, hydrogenis produced by the photoelectrochemical device. In other embodiments,the photoelectrochemical device may produce fuel such as a hydrocarbon(e.g., methane, ethane, propane) and/or a product from the reduction ofcarbon monoxide or carbon dioxide. Other fuels and oxidants can be usedto produce oxygen and a second product, as will be known to those ofordinary skill in the art.

Protons may be provided to the devices described herein using anysuitable proton source, as will be known to those of ordinary skill inthe art. The proton source may be any molecule or chemical which iscapable of supplying a proton, for example, H⁺, H₃₀ ⁺, NH₄ ⁺, etc. Ahydrogen source (e.g., for use as a fuel in a fuel cell) may be anysubstance, compound, or solution including hydrogen such as, forexample, hydrogen gas, a hydrogen rich gas, natural gas, etc. The oxygengas provided to a device may or may not be substantially pure. Forexample, in some cases, any substance, compound or solution includingoxygen may be provided, such as, an oxygen rich gas, air, etc.

The fuel may be supplied to and/or removed from a device and/or systemusing a fuel transport device. The nature of the fuel delivery may varywith the type of fuel and/or the type of device. For example, solid,liquid, and gaseous fuels may all be introduced in different manners.The fuel transport device may be a gas or liquid conduit such as a pipeor hose which delivers or removes fuel, such as hydrogen gas or methane,from the electrochemical device and/or from the fuel storage device.Alternatively, the device may comprise a movable gas or liquid storagecontainer, such as a gas or liquid tank, which may be physically removedfrom the device after the container is filled with fuel. If the devicecomprises a container, then the device may be used as both the fuelstorage device while it remains attached to the electrochemical device,and as a container to remove fuel from the photoelectrochemical device.Those of ordinary skill in the art will be aware of systems, methods,and/or techniques for supplying and/or removing fuel from a device orsystem.

A variety of definitions are now provided which may aid in understandingvarious aspects of the invention.

In general, the term “aliphatic,” as used herein, includes bothsaturated and unsaturated, straight chain (i.e., unbranched) or branchedaliphatic hydrocarbons, which are optionally substituted with one ormore functional groups, as defined below. As will be appreciated by oneof ordinary skill in the art, “aliphatic” is intended herein to include,but is not limited to, alkyl, alkenyl, alkynyl moieties. Illustrativealiphatic groups thus include, but are not limited to, for example,methyl, ethyl, n-propyl, isopropyl, allyl, n-butyl, sec-butyl, isobutyl,tert-butyl, n-pentyl, sec-pentyl, isopentyl, tert-pentyl, n-hexyl,sec-hexyl, moieties and the like, which again, may bear one or moresubstituents, as previously defined.

As used herein, the term “alkyl” is given its ordinary meaning in theart and may include saturated aliphatic groups, including straight-chainalkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic)groups, alkyl substituted cycloalkyl groups, and cycloalkyl substitutedalkyl groups. An analogous convention applies to other generic termssuch as “alkenyl,” “alkynyl,” and the like. Furthermore, as used herein,the terms “alkyl,” “alkenyl,” “alkynyl,” and the like encompass bothsubstituted and unsubstituted groups.

In some embodiments, a straight chain or branched chain alkyl may have30 or fewer carbon atoms in its backbone, and, in some cases, 20 orfewer. In some embodiments, a straight chain or branched chain alkyl has12 or fewer carbon atoms in its backbone (e.g., C₁-C₁₂ for straightchain, C₃-C₁₂ for branched chain), has 6 or fewer, or has 4 or fewer.Likewise, cycloalkyls have from 3-10 carbon atoms in their ringstructure or from 5, 6 or 7 carbons in the ring structure. Examples ofalkyl groups include, but are not limited to, methyl, ethyl, propyl,isopropyl, cyclopropyl, butyl, isobutyl, tert-butyl, cyclobutyl, hexyl,cyclochexyl, and the like. In some cases, the alkyl group might not becyclic. Examples of non-cyclic alkyl include, but are not limited to,methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, n-pentyl,neopentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, and dodecyl.

The terms “alkenyl” and “alkynyl” refer to unsaturated aliphatic groupsanalogous in length and possible substitution to the alkyls describedabove, but that contain at least one double or triple bond respectively.Alkenyl groups include, but are not limited to, for example, ethenyl,propenyl, butenyl, 1-methyl-2-buten-1-yl, and the like. Non-limitingexamples of alkynyl groups include ethynyl, 2-propynyl (propargyl),1-propynyl, and the like.

The terms “heteroalkenyl” and “heteroalkynyl” refer to unsaturatedaliphatic groups analogous in length and possible substitution to theheteroalkyls described above, but that contain at least one double ortriple bond respectively.

As used herein, the term “halogen” or “halide” designates —F, —Cl, —Br,or I.

The term “aryl” refers to aromatic carbocyclic groups, optionallysubstituted, having a single ring (e.g., phenyl), multiple rings (e.g.,biphenyl), or multiple fused rings in which at least one is aromatic(e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl).That is, at least one ring may have a conjugated Pi electron system,while other, adjoining rings can be cycloalkyls, cycloalkenyls,cycloalkynyls, aryls, and/or heterocycyls. The aryl group may beoptionally substituted, as described herein. “Carbocyclic aryl groups”refer to aryl groups wherein the ring atoms on the aromatic ring arecarbon atoms. Carbocyclic aryl groups include monocyclic carbocyclicaryl groups and polycyclic or fused compounds (e.g., two or moreadjacent ring atoms are common to two adjoining rings) such as naphthylgroup. Non-limiting examples of aryl groups include phenyl, naphthyl,tetrahydronaphthyl, indanyl, indenyl and the like.

The terms “heteroaryl” refers to aryl groups comprising at least oneheteroatom as a ring atom, such as a heterocycle. Non-limiting examplesof heteroaryl groups include pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl,pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl,oxadiazolyl, thiophenyl, furanyl, quinolinyl, isoquinolinyl, and thelike.

It will also be appreciated that aryl and heteroaryl moieties, asdefined herein, may be attached via an aliphatic, alicyclic,heteroaliphatic, heteroalicyclic, alkyl or heteroalkyl moiety and thusalso include -(aliphatic)aryl, -(heteroaliphatic)aryl,-(aliphatic)heteroaryl, -(heteroaliphatic)heteroaryl, -(alkyl)aryl,-(heteroalkyl)aryl, -(heteroalkyl)aryl, and -(heteroalkyl)-heteroarylmoieties. Thus, as used herein, the phrases “aryl or heteroaryl” and“aryl, heteroaryl, (aliphatic)aryl, -(heteroaliphatic)aryl,-(aliphatic)heteroaryl, -(heteroaliphatic)heteroaryl, -(alkyl)aryl,-(heteroalkyl)aryl, -(heteroalkyl)aryl, and -(heteroalkyl)heteroaryl”are interchangeable.

Any of the above groups may be optionally substituted. As used herein,the term “substituted” is contemplated to include all permissiblesubstituents of organic compounds, “permissible” being in the context ofthe chemical rules of valence known to those of ordinary skill in theart. It will be understood that “substituted” also includes that thesubstitution results in a stable compound, e.g., which does notspontaneously undergo transformation such as by rearrangement,cyclization, elimination, etc. In some cases, “substituted” maygenerally refer to replacement of a hydrogen with a substituent asdescribed herein. However, “substituted,” as used herein, does notencompass replacement and/or alteration of a key functional group bywhich a molecule is identified, e.g., such that the “substituted”functional group becomes, through substitution, a different functionalgroup. For example, a “substituted phenyl group” must still comprise thephenyl moiety and can not be modified by substitution, in thisdefinition, to become, e.g., a pyridine ring. In a broad aspect, thepermissible substituents include acyclic and cyclic, branched andunbranched, carbocyclic and heterocyclic, aromatic and nonaromaticsubstituents of organic compounds. Illustrative substituents include,for example, those described herein. The permissible substituents can beone or more and the same or different for appropriate organic compounds.For purposes of this invention, the heteroatoms such as nitrogen mayhave hydrogen substituents and/or any permissible substituents oforganic compounds described herein which satisfy the valencies of theheteroatoms.

Examples of substituents include, but are not limited to, aliphatic,alicyclic, heteroaliphatic, heteroalicyclic, halogen, azide, alkyl,aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro,sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl,silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl,sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic orheteroaromatic moieties, —CF₃, —CN, aryl, aryloxy, perhaloalkoxy,aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy,azido, amino, halide, alkylthio, oxo, acylalkyl, carboxy esters,-carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl,alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl,-carboxamidoalkylaryl, -carboxamidoaryl, hydroxyalkyl, haloalkyl,alkylaminoalkylcarboxy-, aminocarboxamidoalkyl-, cyano, alkoxyalkyl,perhaloalkyl, arylalkyloxyalkyl, (e.g., SO₄(R′)₂), a phosphate (e.g.,PO₄(R′)₃), a silane (e.g., Si(R′)₄), a urethane (e.g., R′O(CO)NHR′), andthe like. Additionally, the substituents may be selected from F, Cl, Br,I, —OH, —NO₂, —CN, —NCO, —CF₃, —CH₂CF₃, —CHCl₂, —CH₂OR_(x),—CH₂CH₂OR_(x), —CH₂N(R_(x))₂, —CH₂SO₂CH₃, —C(O)R_(x), —CO₂(R_(x)),—CON(R_(x))₂, —OC(O)R_(x), —C(O)OC(O)R_(x), —OCO₂R_(x), —OCON(R_(x))₂,—N(R_(x))₂, —S(O)₂R_(x), —OCO₂R_(x), —NR_(X)(CO)R_(X),—NR_(x)(CO)N(R_(x))₂, wherein each occurrence of R_(x) independentlyincludes, but is not limited to, H, aliphatic, alicyclic,heteroaliphatic, heteroalicyclic, aryl, heteroaryl, alkylaryl, oralkylheteroaryl, wherein any of the aliphatic, alicyclic,heteroaliphatic, heteroalicyclic, alkylaryl, or alkylheteroarylsubstituents described above and herein may be substituted orunsubstituted, branched or unbranched, cyclic or acyclic, and whereinany of the aryl or heteroaryl substituents described above and hereinmay be substituted or unsubstituted.

The following references are herein incorporated by reference: U.S.Provisional Patent Application Ser. No. 61/103,898, filed Oct. 8, 2008,entitled “Catalyst Compositions and Photoanodes for PhotosynthesisReplication and Other Photoelectrochemical Techniques,” by Nocera, etal., U.S. Provisional Patent Application Ser. No. 61/218,006, filed Jun.17, 2009, entitled “Catalytic Materials, Photoanodes, and Systems forWater Electrolysis and Other Electrochemical Techniques,” by Nocera, etal., U.S. Provisional Patent Application Ser. No. 61/103,905, filed Oct.8, 2008, entitled “Catalyst Compositions and Photoanodes forPhotosynthesis Replication and Other Photoelectrochemical Techniques,”by Nocera, et al., U.S. Provisional Patent Application Ser. No.61/187,995, filed Jun. 17, 2009, entitled “Catalytic Materials,Photoanodes, and Systems for Water Electrolysis and OtherElectrochemical Techniques,” by Nocera, et al., U.S. Provisional PatentApplication Ser. No. 61/073,701, filed Jun. 18, 2008, entitled “CatalystCompositions and Electrodes for Photosynthesis Replication and OtherElectrochemical Techniques,” by Nocera, et al., U.S. Provisional PatentApplication Ser. No. 61/084,948, filed Jul. 30, 2008, entitled “CatalystCompositions and Electrodes for Photosynthesis Replication and OtherElectrochemical Techniques,” by Nocera, et al., U.S. Provisional PatentApplication Ser. No. 61/103,879, filed Oct. 8, 2008, entitled “CatalystCompositions and Electrodes for Photosynthesis Replication and OtherElectrochemical Techniques,” by Nocera, et al., U.S. Provisional PatentApplication Ser. No. 61/146,484, filed Jan. 22, 2009, entitled “CatalystCompositions and Electrodes for Photosynthesis Replication and OtherElectrochemical Techniques,” by Nocera, et al., U.S. Provisional PatentApplication Ser. No. 61/179,581, filed May 19, 2009, entitled “CatalystCompositions and Electrodes for Photosynthesis Replication and OtherElectrochemical Techniques,” by Nocera, et al., and U.S. patentapplication Ser. No. 12/486,694, filed Jun. 17, 2009, entitled“Catalytic Materials, Electrodes, and Systems for Water Electrolysis andOther Electrochemical Techniques.”

The following examples are intended to illustrate certain embodiments ofthe present invention, but are not to be construed as limiting and donot exemplify the full scope of the invention.

Example 1

The following example describes non-limiting examples of methods fordeposition of a catalytic material comprising cobalt (Co-OEC) onto aphotoactive material (e.g., a semiconductor, CdS). The method comprises,in this embodiment, providing a solution comprising metal ionic speciesand anionic species, providing a photoactive electrode, and causing themetal ionic species and the anionic species to form a catalytic materialassociated with the photoactive electrode by application of a voltage(e.g., by an external power source or by exposure to a light source) tothe photoactive electrode.

Materials. Cadmium sulfate, thiourea, ammonium acetate, ammoniumhydroxide solution (28% NH₃), cobalt nitrate, methylphosphonic acid(Aldrich) and fluorine-doped tin oxide (FTO) coated glass substrate(Solaronix) were used as received.

CdS Film Preparation. Thin films of CdS were prepared on FTO-coatedglass substrates by the chemical bath deposition technique. AnErlenmeyer flask containing 100 mL of deionized, distilled water (ddH₂O)was placed in water bath and heated to 88° C. Two 2.5×5 cm FTO-coatedglass substrates were placed in the bottom of the flask with the FTOface up. Cadmium sulfate (0.5 mM), ammonium acetate (10 mM), andammonium hydroxide (0.4 M) were then added to the flask. After 10minutes, four aliquots of thiourea were added to the flask to a finalconcentration of 0.975 mM with 10 minutes between aliquot additions. Tenminutes after the addition of the final thiourea aliquot, the substrateswere removed from the bath and rinsed with ddH₂O. The entire procedurewas repeated four times to yield substantially thick CdS films.

Electrodeposition of Co-OEC on CdS films. The CdS film prepared onFTO-coated glass substrate (e.g., a photoactive electrode) was connectedto a potentiostat (CH Instruments 760C) via an alligator clip andimmersed in water containing 2 mM cobalt nitrate and 0.1 Mmethylphosphonic acid (pH 8.5) (e.g., a solution comprising metal ionicspecies and anionic species). An Ag/AgCl reference electrode (BASi) andplatinum wire counter electrode were connected to the potentiostat andimmersed in the solution. The CdS electrode was biased at 1.5 V vs.Ag/AgCl for one hour (application of a voltage to the photoactiveelectrode using an external power source). The CdS electrode was thenremoved from solution and rinsed with deionized water. FIG. 14 shows ascanning electron micrograph (SEM) of the resulting electrode. The darkmaterial on top is the resulting Co-OEC catalytic material (e.g.,catalytic material associated with the photoactive electrode comprisingmetal ionic species and anionic species) that has been electrodepositedonto the CdS semiconductor underneath. As shown in FIG. 14, a largeportion of the Co-OEC overlayer has flaked away during drying of theelectrode for SEM, revealing the CdS film underneath. Electrondispersive x-ray (EDX) analysis confirms the presence of Co, Cd, S, andP.

Photodeposition of Co-OEC on CdS. The CdS film prepared on FTO-coatedglass substrate was connected to a potentiostat (CH Instruments 760C)via an alligator clip and immersed in water containing 2 mM cobaltnitrate and 0.1 M methylphosphonic acid (pH 8.5). An Ag/AgCl referenceelectrode (BASi) and platinum wire counter electrode were connected tothe potentiostat and immersed in the solution. The electrode was held at0.5 V vs. Ag/AgCl and illuminated for one hour with light (e.g.,application of a voltage using an external light source) from a 300 W Xearc lamp equipped with a 495 nm long pass filter (λ>495 nm) and a 0.8 AUneutral density filter. The electrode was then removed from solution andrinsed with ddH₂O. FIG. 15 shows an SEM of the resulting electrode, bothfor regions that were exposed to light (FIG. 15A) and maintained in thedark (FIG. 15B). The illuminated portion of the film (FIG. 15A) exhibitsa cracked morphology, which may be attribute to drying of the resultingCo-OEC overlayer coating. The region of film that was not exposed tolight did not exhibit this cracking morphology and, instead, shows auniform CdS film. EDX analysis confirms the presence of Co, Cd, S, and Pfor the film exposed to light, while films kept in the dark lackedmeasurable diffraction peaks for Co and P.

Example 2

The following prophetic example describes methods for formation of aCo-OEC functionalized photoanode and characterization of the enhancedphotoassisted water oxidation reaction rate.

Nanostructured iron oxide semiconductor (α-Fe₂O₃) may be grown onelectrically conductive FTO-coated glass substrates by the atmosphericchemical vapor deposition (CVD) technique as described previously (e.g.,See Kay et al., J. Am. Chem. Soc, 2006, 128, 15714-15721). The substratemay then be attached to a potentiostat as the working electrode andimmersed in a solution of 0.1 M KPi (pH 7) and 0.5 mM Co(NO₃)₂. Theelectrode may then be biased at 1.1 V vs. Ag/AgCl reference for theelectrodeposition of the Co-OEC catalyst as described in Example 1 andas done previously on ITO electrodes (e.g., see Kanan et al., Science,2008, 321, 1072). The resulting α-Fe₂O₃/Co-OEC electrode may then serveas a photoanode.

The α-Fe₂O₃/Co-OEC photoanode may exhibit an enhanced rate forphotoassisted water oxidation, compared to the α-Fe₂O₃ photoanode alone.Photoanodes may be immersed in 1 M NaOH aqueous electrolyte, along withan Ag/AgCl reference and Pt wire counter electrode. The photoanode maythen be illuminated with AM 1.5 simulated solar irradiation and a biasapplied and swept from −0.2 to 0.6 V vs. Ag/AgCl reference. In thisexperiment, the photocurrent onset potential is the applied biaspotential at which the photoanode exhibits a measurable anodic(oxidative) current and is familiar to those skilled in the art. Theonset potential may be observed to shift to less positive values for theα-Fe₂O₃/Co-OEC photoanode compared to α-Fe₂O₃ alone, owing to thecatalytic effect of the Co-OEC water oxidation catalyst. Additionally,the overall magnitude of the anodic photocurrent may be larger for theα-Fe₂O₃/Co-OEC photoanode compared to α-Fe₂O₃ alone. An incidentphoton-to-current efficiency (IPCE) measurement may then be carried out,in which the photon-to-current conversion efficiency is measured as afunction of excitation wavelength. Devices and methods for measuring theIPCE will be known to those of ordinary skilled in the art. The IPCE maybe shown to increase by some value (e.g., at least about 50%, at leastabout 100%, at least about 200%, etc.) as a function of excitationwavelength for the α-Fe₂O₃/Co-OEC photoanode compared to α-Fe₂O₃ alone.

Example 3

The following prophetic example describes non-limiting methods for wateroxidation, O₂ gas evolution, and detection using Co-OEC functionalizedphotoanodes.

A Co-OEC functionalized photoanode (e.g., as prepared according toExample 1 or 2, or otherwise as described herein) may be attached to apotentiostat and serves as the working electrode for this experiment.The working electrode may be immersed in a buffered aqueous solution(e.g., 1 M KPi, pH 7) along with a reference electrode (e.g., Ag/AgCl)and an auxiliary electrode (e.g., Pt wire). The entire experiment maythen be sealed from the environment (e.g., using rubber septa in groundglass joints attached to the electrochemical cell housing) and purged ofair by bubbling with He gas (or other inert gas, e.g., N₂, Ar). Thephotoanode may then be biased at some potential relative to thereference electrode (e.g., 0<E<1.5 V). The photoanode may then beilluminated with light (e.g., from a Xe arc lamp that may or may not befiltered to produce solar AM 1.5 radiation) through a transparent (e.g.,quartz) window in the reaction vessel. The light may or may not passthrough the back side of the photoanode, such that the semiconductor isilluminated first prior to illumination of the Co-OEC film. Anodicphotocurrents may be measured with the potentiostat. Bubbles may or maynot be visible at the photoanode. Gaseous products of thephotoelectrochemical reaction may then be analyzed by withdrawingsamples of the reaction headspace using a gas-tight syringe andinjecting the sample into a gas chromatograph/mass spectrometer. Thedetection of a peak with m/z=32 should indicate the production of O₂.This may be confirmed by operation of the photoelectrochemical cell inwater containing some fraction of H₂ ¹⁸O and with the detection of peakswith m/z=₃₄ (^(18,16)O₂) and m/z=36 (18,18O₂). Gaseous oxygen may alsobe detected and quantified using a phosphorescence-based O₂ sensingprobe (e.g., FOXY, Ocean Optics).

Control experiments may also be performed. The same semiconductorphotoanode minus the Co-OEC catalytic material may be tested forphotoelectrochemical water oxidation, the photocurrents measured, andthe O₂ quantified with similar methods. The catalytic effect of Co-OECshould yield higher photocurrents and larger amounts of O₂ produced perunit time for the Co-OEC/semiconductor photoanode compared to thesemiconductor alone.

Example 4

The following prophetic example describes non-limiting methods forfabricating a Co-OEC functionalized tandem photoelectrochemical cell(e.g., n-CdS/n-TiO₂).

A tandem photoelectrochemical cell composed of n-CdS/n-TiO₂ may befabricated as previously described (e.g., See Nakato et al., Nature,1982, 295, 312-313). The cell is composed of a sandwich of one n-TiO₂wafer and one n-CdS wafer. The sandwich houses a solution of 1 M NaOH, 1M Na₂S, and 1.5 gram atom 1⁻¹ S, which is maintained by epoxy sealant onthe edges of the wafers. The outer face of the CdS wafer is attached toa copper wire via an indium metal contact. The outer face of the TiO₂wafer is exposed to solution as the surface for water oxidation.

The tandem PEC thus described may then be functionalized with Co-OECusing a photodeposition procedure, for example, as described inExample 1. In particular, the n-CdS/n-TiO₂ photoelectrode may beimmersed in the anode compartment of a two-compartmentphotoelectrochemical cell containing an aqueous solution of 0.5 mMCo(NO₃)₂ and 0.1 M KPi (pH 7). The copper wire from the CdS wafer may beattached to a platinum gauze electrode immersed in the cathodecompartment of the cell, which may contain 0.1 M KPi (pH 7). The anodeand cathode compartments may be separated by a glass frit or a membrane.Excitation of the anode with light of wavelength less than or equal to400 nm may then effect the deposition of the Co-OEC catalyst on theouter surface of the TiO₂ wafer. The photolysis time may correlate withthe thickness of the Co-OEC film. The photocurrent may or may not beobserved to rise with photolysis time.

The Co-OEC functionalized tandem PEC may then be tested for enhancedphotoassisted water oxidation as described in Example 2 and for O₂evolution, for example, as described in Example 3.

Example 5

The following prophetic examples described a non-limiting method forfabricating and testing a Co-OEC functionalized tandemphotovoltaic-photoelectrochemical device (e.g., p,n-GaAs/p-GaInP₂).

A device may be fabricated in which a p-GaInP₂ photocathode is grown ontop of a p,n-GaAs photovoltaic, as previously described (e.g., SeeKhaselev et al., Science, 1998, 280, 425-427). The device may beelectrically connected to conductive anode support (e.g., Pt, FTO, Ni).The device may be immersed in electrolyte within a two-compartment cell.The anode compartment may contain the conductive anode support, 0.1 MKPi buffer (pH 7), and 0.5 mM Co(NO₃)₂. The cathode compartment maycontain the p,n-GaAs/p-GaInP₂ device and 0.1 M KPi buffer (pH 7). Thetwo compartments may be separated by a glass frit. Illumination of thep,n-GaAs/p-GaInP₂ device may produce a current and initiate depositionof the Co-OEC catalyst on the anode. The photocurrents may be observedto increase with Co-OEC layer thickness due to the catalytic effect ofCo-OEC. The Co-OEC functionalized tandemphotovoltaic-photoelectrochemical device may then be tested for O₂evolution, for example, as described in Example 3 with the exceptionthat in this case the cathode is illuminated with light, rather than theanode.

Example 6

The following prophetic example describes a non-limiting method forfabricating and testing of a Co-OEC functionalized dye-sensitizedphotoanode. In this embodiment, the method comprises providing asolution comprising metal ionic species and anionic species, providing aphotoactive electrode comprising a photoactive composition and aphotosensitizing agent, and causing the metal ionic species and theanionic species to form a composition associated with the photoactiveelectrode by application of a voltage to the photoactive electrode.

Mesoporous titanium dioxide (TiO₂) films (e.g., photoactive composition)may be prepared on a conducting glass substrate (e.g., FTO glass) andRuL₃ (L=2,2′-bipyridine-4,4′-dicarboxylic acid) dye (e.g.,photosensitizing agent) may be adsorbed to the TiO₂ film as describedpreviously (e.g., see O'Regan et al., J. Phys. Chem., 1990, 94,8720-8726). Thus formed, the dye-sensitized photoanode (e.g.,photoactive electrode) may then be attached to the working electrode ofa potentiostat and immersed in a two-compartment cell. The anodecompartment may contain the dye-sensitized photoanode, an Ag/AgClreference electrode, 0.5 mM Co(NO₃)₂, and 0.1 M KPi buffer (pH 7) (e.g.,solution comprising metal ionic species and anionic species). Thecathode compartment may contain hydrogen evolution electrode (e.g., aPt-wire). The Co-OEC catalyst may then be electro- or photodeposited atthe anode (e.g., composition associated with the photoactive electrode),for example, as described in Example 1 (e.g., by application of voltageto the photoactive electrode, via an external power source or via alight source). The device may then be tested for enhanced photoassistedwater oxidation, (e.g., as described in Example 2) and photochemical O₂evolution (e.g., as described in Example 3).

Example 7

The following prophetic example describes a non-limiting example of aphotoanode comprising a band-gap engineered titanium dioxidesemiconductor.

Titanium dioxide (TiO₂) has a band-gap of 3.0 eV, thus limiting itsabsorption to UV light (<2% of the solar spectrum). Significant researchhas focused on the engineering of this metal oxide semiconductor tolower its band-gap and allow for the absorption of visible light. Forexample, TiO₂ has been doped with nitrogen, carbon, and sulfur atoms, toraise the energy of the valence band (e.g., see Asahi et al., Science,2001, 293, 269-271). In most embodiments, absorbed red photons do notcontribute to substantial photocurrents, thus the overall efficiency ofthese materials for solar powered water oxidation remains low. Themechanism of water oxidation at TiO₂ surfaces may involve the formationof 1-electron oxidized, high energy intermediates (i.e. hydroxylradicals, .OH), thus owing to the inability of TiO₂ to support the4-electron hole (4 h⁺) catalysis of water oxidation. Upon doping, thevalence band electron-holes generally lack the oxidative power toproduce .OH (FIG. 16), thus shutting down water oxidation chemistry. Incontrast, the Co-OEC catalyst may be found to operate near the4-electron/hole potential for water oxidation with low overpotential.FIG. 16 shows the band edge positions of various forms of TiO₂ alongwith the standard reduction potential of the hydroxyl radical and thepotential for operation of the Co-OECcatalyst. Photogeneratedelectron-holes in band-gap engineered TiO₂ may thus possess sufficientenergy to oxidize any Co-OEC adsorbed at the surface. Thus, depositionof a thin film of Co-OEC on band-gap engineered TiO₂ may engenderphotochemical water oxidation activity in this otherwise inactivematerial.

Thin films of nitrogen-doped TiO₂ may be prepared by sputtering,sol-gel, and solution anodization of Ti. Co-OEC films may then be formedby electro- and photodeposition methods, as described herein. Incidentphoton-to-current efficiency (IPCE) may be measured (e.g., as describedherein) to test for enhanced photochemical response of the doped TiO₂upon adsorption of the Co. Undoped TiO₂ films, which show no IPCEresponse for visible wavelength excitation, may be used as a control.

Example 8

The following prophetic example describes the stabilization of softphotoactive semiconductors towards photocorrosion in aqueous media.

Electron-holes photogenerated in the valence band of soft n-typesemiconductors may diffuse to the semiconductor-electrolyte interfacewhere they initiate corrosion of the material. The formation of ahole-tunneling layer on the surface of the soft n-type semiconductor mayprevent oxidation of the semiconductor lattice and photocorrosion. TheCo-OEC catalyst may then be deposited on the hole-tunneling layer.Tunneling of the electron-hole out of the semiconductor valence band,through the hole-tunneling layer, and into surface-adsorbed Co-OEC mayprovide a mechanism for water oxidation that circumvents photoanodecorrosion. The hole tunneling layer material may be anothersemiconductor material (e.g., TiO₂) and may be chosen such that thevalence band of the hole tunneling layer is more positive in potentialenergy with respect to the valence band of underlying soft, photoactivesemiconductor (e.g., CdS).

CdS films may be prepared by the chemical bath technique described inExample 1. TiO₂ films may then be prepared over the CdS film by standardsputtering, sol-gel, or electrodeposition techniques. Co-OEC catalystfilms may then be photo- and/or electrodeposited from aqueous solutionsof Co(II) ions as described in Example 1. The TiO₂ films may be preparedsuch that they are thin enough (e.g., ˜10 nm thick) to allow forefficient tunneling of the electron hole from the underlying CdS layerinto the Co-OEC film. Electrode photostability may be characterized as afunction of light intensity, catalyst deposition conditions, thickness,and/or substrate morphologies. Materials that are photoactive towardoxygen evolution and produce stable photocurrents on long timescales maybe further optimized.

Example 9

The following prophetic example describes non-limiting designs ofphotoelectrochemical cells.

A single band gap device (FIG. 17A) contains a single light-absorbingsemiconductor material. The conduction band electrons and valence bandholes produced upon excitation have suitable energy for proton reductionand water oxidation, respectively. The valence band holes aretransported into the adsorbed CoPi catalyst, while the electrons may bedirected to the cathode catalyst either coated onto a separate electrode(as shown), or deposited onto the back of the ohmic contact of thesemiconductor. The first method allows for collection of electrons andgeneration of hydrogen at a remote site, while the second method affordshydrogen evolution over the surface area of the photoanode. A range ofcathode catalysts may be employed, including thin monolayers of Pt(e.g., deposited from solutions of H₂PtCl₆) or Pt on C and alloys of Cu,Mo, and Ni. Materials may be selected based on overall deviceperformance and cost.

If the conduction band electrons do not have sufficient energy forproton reduction, a bias voltage may be required to perform the watersplitting reaction. In FIG. 17B, this voltage is supplied by ap:n-junction PV stacked in series with the photoanode semiconductor. Inthis tandem configuration, blue photons are absorbed by the photoanodeand red photons are transmitted to the PV. The current collected by theohmic contact may then be channeled to the cathode electrode forhydrogen evolution.

FIG. 17C depicts another type of tandem PEC, in which the PV has beenreplaced by a p-type semiconductor photocathode. Electron-holes aredirected to the semiconductor-electrolyte interface for n-typesemiconductors, and, similarly, electrons are direct to the surface ofp-type semiconductors where they may be collected by an adsorbed layerof cathode catalyst for hydrogen evolution. Photocathodes may becomprised of p-CdS, p-Si, and p-Fe₂O₃, among others.

In some cases, PEC modules may also be produced on dimensions similar tothat for commercial PVs (˜1 m²). In a non-limiting design, as shown inFIG. 17D, a PEC system is housed within glass or plexiglass domes 218that focus the light onto a semiconducting material coated with CoPi(e.g., 212) below. Oxygen (white circles) may evolve from thesemiconductor/CoPi 212, while hydrogen (black circles) may evolve from astainless steel or conducting plastic cathode (e.g., 208) coated with acathode catalyst. The gases may be collected at the top of the anode andcathode compartments, which can be separated by gas-impermeable,ion-conductive membranes (e.g., 210). The device may comprise one ormore electrolytes (e.g., 214, 216, generally aqueous) in the anode andcathode compartments. This device may require that the cost per unitarea of membrane is lower than for the semiconductor/CoPi photoanode.

Alternatively, the membrane may be incorporated into the PECsemiconductor. FIG. 17E shows a tandem PEC in which holes have beendrilled and filled with the membrane material. Oxygen may be generatedfrom the photoanode and released into the atmosphere, while hydrogen maybe generated at the photocathode underneath and trapped at the apex ofthe device. The gases may also be produced by collecting the electrodesthe photocathode and concentrating them at a separate electrodecompartment as shown in FIG. 17F. An ion-permeable membrane may providecontact between the anode and cathode compartment while keeping thehydrogen and oxygen gases separate.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, the invention may be practiced otherwise than asspecifically described and claimed. The present invention is directed toeach individual feature, system, article, material, kit, and/or methoddescribed herein. In addition, any combination of two or more suchfeatures, systems, articles, materials, kits, and/or methods, if suchfeatures, systems, articles, materials, kits, and/or methods are notmutually inconsistent, is included within the scope of the presentinvention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

1. A method for forming a photoanode for the catalytic production ofoxygen from water, comprising: providing a solution comprising metalionic species and anionic species; providing a photoactive electrodecomprising a photoactive composition and a photosensitizing agent; andcausing the metal ionic species and the anionic species to form acatalytic material associated with the photoactive electrode byapplication of a voltage to the photoactive electrode.
 2. A method forproducing oxygen from water, comprising the steps of: providing aphotoelectrochemical cell comprising: a photoactive electrode comprisinga photoactive composition and a photosensitizing agent; an electrolyte;and a catalytic material integrally connected to the photosensitizingagent, the catalytic material comprising metal ionic species and anionicspecies, and wherein the catalytic material does not consist essentiallyof a metal oxide or metal hydroxide; and illuminating thephotoelectrochemical cell with light to thereby produce oxygen gas fromwater. 3-9. (canceled)
 10. The method of claim 1, wherein the voltage isapplied to the photoactive electrode by a power source.
 11. (canceled)12. The method of claim 1, wherein the voltage is applied to thephotoactive electrode by exposing the photoactive electrode toelectromagnetic radiation. 13-33. (canceled)
 34. The method of claim 1,wherein the anionic species comprises phosphorus.
 35. The method ofclaim 1, wherein the metal ionic species comprises cobalt ions. 36.(canceled)
 37. The method of claim 1, wherein the metal ionic specieswith an oxidation state of (n+x) and the anionic species define asubstantially non-crystalline composition and have a K_(sp) value whichis less, by a factor of at least 10³, than the K_(sp) value of acomposition comprising the metal ionic species with an oxidation stateof (n) and the anionic species.
 38. (canceled)
 39. The method of claim2, wherein the photoelectrochemical cell further comprises a secondelectrode. 40-41. (canceled)
 42. The method of claim 39, wherein thesecond electrode is a photoactive electrode. 43-49. (canceled)
 50. Themethod of claim 2, wherein the water contains at least one impurity51-89. (canceled)
 90. The method of claim 1, wherein thephotosensitizing agent comprises a metal complex dye.
 91. The method ofclaim 1, wherein the photosensitizing agent comprises an organic dye.92-107. (canceled)
 108. A photoanode for the catalytic production ofoxygen from water, comprising: a photoactive electrode comprising aphotosensitizing agent and a photoactive composition; and a catalyticmaterial comprising metal ionic species and anionic species, wherein thecatalytic material is formed by application of a voltage to aphotoactive electrode.
 109. The photoanode of claim 108, wherein themetal ionic species comprises cobalt ions.
 110. The photoanode of claim108, wherein the anionic species comprises phosphorus. 111-162.(canceled)
 163. The photoanode of claim 108, wherein the metal ionicspecies comprise at least a first and a second type of metal ionicspecies. 164-169. (canceled)
 170. The photoanode of claim 108, whereinthe photosensitizing agent comprises a metal complex dye.
 171. Thephotoanode of claim 108, wherein the photosensitizing agent comprises anorganic dye. 172-234. (canceled)
 235. The method of claim 2, wherein theanionic species comprises phosphorus.
 236. The method of claim 2,wherein the metal ionic species comprises cobalt ions.